Six days on the road, and 330 miles north of Carlisle, you begin to despise the Highlands and start yearning for the wide open flatlands of Norfolk. You have been pedalling uphill forever; well, for a good hour or so, struggling to maintain a steady seven mph against a wild west wind. The gale is whistling straight from Nova Scotia to the ancient granite mountains, riven by glaciers and tarred by 1980s roadbuilders. A yard to your right, 40-ton trucks are hurtling past at 10 times your velocity. The gutter into which you are forced to steer is littered every 20 yards or so by a corpse of some creature even lower along the evolutionary scale than a cyclist.

Drumochter Pass is one of the highest and most barren passes in Scotland. These days, it carries a high-speed section of the A9 between Blair Atholl and Aviemore. Yet it is part of the official slow road through Scotland, 402 miles from the border to Inverness.

Fortunately, the guide to the Carlisle to Inverness cycle route assures you that these seven deadly miles comprise the worst stretch of the entire ride. The transport charity Sustrans, which has developed the route as part of the National Cycle Network, endeavours to keep cyclists separate from cars for the mutual benefit of both parties. And mostly, the plan succeeds in keeping you on byways, not highways. If you wondered what happened to old A-roads when they were rendered obsolete by expressways, the Sustrans route has the answers. You develop an intense fondness for the old A9, a meandering lane now largely abandoned to the weeds and the cyclists, and refreshingly clear of decomposing wildlife.

The Sustrans trail begins safely enough, too, on a pedestrian precinct outside the old town hall in Carlisle. Your passage to Scotland begins irritatingly slowly, since you are not actually allowed to cycle for the first half-mile - against the traffic along a one-way street. You then spend ages shrugging off the suburbs of Carlisle, before you climb to Gretna. No sign by the cycleway announces your arrival into Scotland; only the presence of coach parties, swarming around the old blacksmith's shop, indicates you have crossed from England to a country composed of lowland and uplands, highlands and islands.

The cartographer in modern Britain is largely constrained to adding double stripes of blue, signifying new motorways, largely through areas of outstanding natural beauty. So the Sustrans maps are true works of art. The designers have divined a secret vein through the heart of Scotland. One minute you can be puffing inelegantly beside an old steam-train line that sweeps gracefully in from a monumental viaduct (Rusko summit; mile 86). The next you are seized by a stray southerly gust and sent spinning around the broad arc of a seaside promenade (Ayr; mile 142). Except that chasing this course through Scotland takes many thousands of minutes.

You need not be supremely fit to cover 400-plus miles in six days - particularly if you do what I did and exercise the option of covering parts of the route by train, knocking over 100 miles out of the reckoning. Although ScotRail seems to employ a platoon of people whose sole job is to prevent people taking bicycles by train, with patience and perseverance you can shelter from the rain and sneak out of a climb or two with a short hop on the railway.

The main point, though, is to revel in a cross section of an entire nation and to pedal through the finest scenery in the British Isles. A week ago I descended from Slochd summit, along the course of General Wade's military road, down through Culloden, then circled, clockwise, the anti-climatic mini-roundabout in Inverness where the route ends.

Yet the journey keeps cycling through my mind with undiminished imagery. You get a powerful cocktail from equal measures of misery and agony, joy and astonishment. The flashbacks arrive rapidly and randomly, always prefixed by the little red number that shows the distance so far. As the days and the miles tick by, you become worryingly adept at subtracting from 402 (the total distance in miles).

Minus 332: Gatehouse of Fleet, buried in the heart of Dumfries and Galloway. A grander town in name than reality; a few hardy cottages crouched around a Victorian clock tower. The only sign of resident life is a fellow cyclist whose exertion is limited to a broad, toothless grin from a crinkly old face. The weather-beaten wrinkles surrounding his smile testify to decades of tackling the hill that stands between you and your first night stop in Newton Stewart. He is happy because, unlike you, his rusting Raleigh is parked next to him, with no evidence of imminent departure in any direction except back to the bar for a refill of his pint.

Any resentment as you climb wearily back on to your bike is washed away by the fast-flowing Big Water of Fleet (names are conjured extravagantly around here), whose valley you follow. Suddenly the stern firs are subverted by a flurry of flowers, bluebells blossoming from the grassy banks in huge, billowing waves. Siphon as much inspiration as you can, for the first long ascent of the trip. Sea level to 800 feet takes six sapping miles, but hoists you through half a dozen habitats until you crown the summit on a plateau of blank moorland that looks as if it has tumbled from the moon.

One day and one climb behind you, many more to come: the brutally steep Nick of the Balloch, short sharp Carrick Hill, the long haul to Slochd Summit. Yet despite the terrain's tendency to slope much more uphill than down, there is no better way to slice through Scotland. From Firth to Firth - Solway to Moray - you carve a meandering course through town and country, city and forest. Somehow, by heading due north, you manage to cross from the West coast to the East. And by the time you breast the summit at Drumochter, you will believe that anything is possible.