David Baxter tries out a new long-distance trail from Cantabria to the Med

Vultures hovered above the towering cliffs. We joked about them picking off any stragglers in our cycling group though, even for a novice like me, the going was not tough.

We were in the green hills of Cantabria, north-east Spain, following the river Ebro from its source through a series of spectacular gorges, the Canyons of the Ebro. At one point we rode (or pushed) our bikes up to the plateau, to be rewarded by giddying views of the turquoise river on its sinuous course.

I decided to follow the Ebro – the longest river that's wholly in Spain – from its source near the Atlantic to its delta in the Mediterranean, some 570 miles. I allowed three weeks and armed myself with a lavishly illustrated guide to the GR99, a new long-distance trail. As it turned out, I rarely used the trail, but I found its frequent information boards strangely comforting.

For the first part of the trip I joined a tour organised by Iberocycle, run by a genial ex-pat Lancastrian, Simon Proffitt. With our bags forwarded daily to the next small but comfortable rural inn, and Simon on hand with support, we could concentrate on covering modest daily distances and enjoying the scenery, tiny somnolent villages and ancient churches.

We passed through historic hilltop towns such as Frias and Oña, then through the last canyon-mouth into the rolling countryside of La Rioja, where we pedalled through vineyards past little domed stone shelters and spectacular modern bodegas. We spent a night in unaccustomed luxury in Haro, the wine-centre of Rioja, before ending our tour in the medieval walled town of Laguardia.

Bikeless now, I switched to buses and trains. At Logroño I shared tapas bars with wine buffs and Camino pilgrims. From here, the Ebro flows on, screened by tall trees, its broad fertile valley increasingly busy with roads and commerce. For centuries the river was the main transport route in the region, fought over by Celtiberians, Romans, Visigoths, Muslims and others. Later it became a key battleground in the Spanish civil war. Nowadays, it provides half Spain's hydroelectricity and enables arable farming to flourish in an otherwise dry region.

Ancient towns line the valley: Calahorra, Alfaro with its storks, Tudela with its maze of streets. Vegetables gradually replace vines. I enjoyed a special menu with white asparagus, beans, and artichokes and lettuces.

Arriving in Zaragoza's imposing Delicias station, I crossed the Ebro in a chairlift – one of four new crossings – giving me a bird's-eye view of the Expo 2008 site. Some Expo buildings are being demolished, others "repurposed", including Zaha Hadid's twisty pavilion bridge. I stayed in the Hiberus hotel, a hymn to modern design. The flurry of sprucing-up prompted by Expo includes promenades, bike paths and beaches along the river, but there's plenty of older stuff, too, from Roman walls to Moorish palaces, as well as the city's medieval Jewish heritage and Renaissance churches.

I moved on to Caspe, the self-styled capital of the Sea of Aragon, a reservoir created by the damming of the Ebro, beloved of anglers and outdoor sports enthusiasts. Then I realised that to meet my deadline I'd have to skip this wild, remote section. I headed for Catalonia's Terra Alta from where a via verde – disused railway line – leads through the mountains to meet the Ebro as it descends to the sea.

I rented a bike for the gentle descent along a path carried by bridges, tunnels and viaducts, through deep gorges. Nearby, the GR99 crosses the river by one of the last Ebro ferries, past the magnificent castle commanding the river in the quiet village of Miravet.

The delta was a surprise. Not a semi-wilderness but intensively farmed, mainly for rice. The central town, Deltebre, is an unattractive sprawl but the wetland reserves are home to more than 300 bird species. Finally, I walked the short distance from the last GR99 marker to complete my trek at the mouth of the Ebro.

Compact Facts

How to get there

Santander and Bilbao are the nearest transport hubs for start of the Ebro trail. Iberocycle's (00 34 942 58 10 85; iberocycle.com) next nine-day guided tour runs from 12 to 20 September, price €940 per person, based on two sharing, plus €88 for bike hire.

Further information

Camino Natural del Ebro: GR99 (ISBN: 9788483204511), £18.95. Spanish Tourist Office (tourspain.co.uk).