London: the step-by-step guide Michael Leapman walks along the Grand Union Canal

the grand union canal, built in 1800 to link London with the industrial Midlands, is under-used by walkers. Its flat waterside paths, not always beautiful but often fascinating, give a unique rear view of buildings normally seen only from the street. Often you can walk for miles along the bank, meeting only a few patient anglers and some ducks. This week's route between tube stations covers part of the canal's Paddington branch.

Beginning among factories and warehouses, it passes neat suburban back gardens to end with an invigorating climb up Horsenden Hill.

Turn right out of Harlesden station, crossing the bridge and admiring its coloured tiled panels - a railway motif on one side and a canal boat on the other. Follow the road downhill and take the second turning on the right, Barretts Green Road. After a few yards, just before a betting shop, turn right down steps to the towpath.

Turn left, keeping the canal on your right. A few geese and mallards lend a faintly rural air but essentially this is factory country and soon you see the 1920s complex of the canners H J Heinz, occupying several acres of the north bank. Until the 1960s, Heinz were still transporting some products by water.

The canal, briefly fringed by trees, now curves attractively to the right. A little further on, it crosses the North Circular Road and the River Brent via an aqueduct, letting you look down on the speeding traffic and the grubby river.

Gradually houses replace factories as the dominant feature of the landscape, and a pair of swans add interest to the wildlife. On the right, the Pleasure Boat pub appears, with a waterside garden that would be tempting in summer. Several canal boats are moored here, awaiting the start of the season.

On the left is a large Sainsbury's supermarket, with moorings for waterborne shoppers. Just beyond it, after passing beneath a brick bridge, the surfaced path ends and you walk on a track of stone and earth that can be muddy in places.

Now the outlook is distinctly greener, with Sudbury golf course on your right and a range of low hills ahead. It is nearly a mile to the next bridge, and immediately in front of it you leave the towpath by steps on the left and turn right to cross the canal into Horsenden Lane North. (If you want to omit the hill climb, stay on the towpath to the next footbridge.)

After 50 yards, take the footpath on your right through a brown metal gate, and fork right, away from the road on to a path that goes uphill through trees. Soon it opens up into a grassy area with coppiced trees. Keep straight on uphill, a view of Greenford on your left, and at a crossing keep ahead on a clear path between brambles.

The next grassed area is bounded by logs laid end to end. Walk through the gap in them and make for the matching gap on the other side. Here the path goes left but, being so near to the top of the hill, it is worth detouring right up to the summit, with its good view of Harrow to the north-east.

Now turn back down the wide grassy path, heading towards rugby fields. When you get to a drive, walk straight across it to a surfaced downhill path and, after 50 yards, turn left to a distinct wooden barrier, crossing Horsenden Lane to a similar barrier opposite and entering the woods.

After ten yards, turn left on a path through the trees. Keep right at the next junction and go up four wooden steps. Further on you descend three steps to reach an open field. Take the path that cuts off its corner, then keep to the right of the trees along the left-hand edge of the field. At the end of the field go left into the trees to cross a narrow stream.

Turn right and follow the edge of the next field to a gap in the trees on your right. This leads to another field, where you stick to the left- hand edge until you come to a low plank bridge on your left, by a post with a white arrow. Cross the bridge and you will soon find yourself on the canal bank.

Turn right and keep as close to the canal as you can, although the path will sometimes take you into the fields. When you get to the footbridge, cross it (to rejoin walkers who stuck to the canal). Keep ahead along the path that crosses wasteland towards Greenford Road. The path curves left to follow the line of the road, making for the raised railway track ahead. Just before the rail bridge, turn right and cross Greenford Road into Rockware Avenue, turning left past the bus stop for the station.

Distance: Four-and-a-half miles

Time: Two-and-a-quarter hours

Transport: Start at Harlesden (Bakerloo Line, zone 3); end at Greenford (Central Line, zone 4)

Caution: some mud in later stages