Leaning my skis against the thick walls of the bomb shelter before I entered its sinister decontamination zones, I mused that, 25 years ago, this was one of the safest places to be in Germany. It’s easy to forget the paranoia of those times, when 300,000 Soviet soldiers stood guard in East Germany, where I was now skiing. And I was aware that I was being followed – not by the Stasi, but by my luggage. Where there was once a ruthlessly efficient secret service, there was now a super-efficient baggage transfer service the length of the Rennsteig, a historic 169km trail along the top of the Thuringian mountains, which bears many testaments to the Cold War era and earlier.

The trail is popular with cyclists, walkers and runners in summer, but there are also 22 small downhill ski resorts leading off the 1,000-metre-high ridge. Growing in popularity is the adventure of exploring the trail on cross-country skis, and for me there couldn’t be a more beautiful and exciting way to go.

On a bright, crisp morning, I set off to follow milestones erected from 1643 onwards from Hotel Schieferhof in the little town of Neuhaus am Rennweg, between Nuremburg and Leipzig.

My guide was a €3 trail map, and I’d booked my hotels and luggage transfers through (German-only) website rennsteig-und-mehr.net, which recommends doing sections of between 18 and 25km a day, though it will organise any distance or duration. And there’s no need for much cross-country skiing experience; the trip can simply be treated as walking or running on skis, depending on your level of fitness.

The going was generally easy, until the times the freshly groomed trail turned into a narrow riverbed that I could scarcely snowplough down. I had to plunge into the fresh powder at the side to slow myself, and eventually resorted to unclipping my skis and walking down.

I regularly got lost, and it was only by asking directions in broken German – for over-40s around here, Russian, not English, is the second language – that I found my way. “Let’s take this route – it’s sunnier,” one fellow skier suggested. And soon I found myself at the observation tower in the little resort of Masserberg.

Climbing to the top, I could see how the mountain range had formed a natural second line of defence for the Soviets and GDR, a few kilometres east of the barbed wire East-West Germany border. And I could also see a hubbub of activity below me. An international mushing championship was in full swing, so as I sped down to the village, I was racing huskies eyeball-to-eyeball. At the bottom the glühwein was flowing freely and the postilion of a horse-drawn sled blew his curly bugle to part the crowds.

I had expected to spend the nights in rather spartan hostels, but at Spa Hotel Auerhahn not only was my bag waiting, but the food and wine were excellent and inexpensive. The next day, reinvigorated, I flew along wide open avenues, using the relatively modern technique of skate skiing, which only works on packed snow. At other times I used the less tiring classic technique down narrower trails, exchanging a friendly “Schi heil” with passing skiers.

At the village of Neustadt am Rennsteig, wood smoke was swirling between typically Thuringian houses, with tiled, curved walls. I carried my skis to the Rennsteig Museum, which tells the history of the trail and how it had been used by messengers since the 14th century. I had by now cracked the Rennsteig’s code, which is to follow the white Rs painted on tree trunks, even when beautifully prepared trails entice you left or right on to the other 1,000km of paths.

I made such good progress I was on time for a tour of the Bunker Museum near Schmiedefeld, where the guide led us past mannequins in gas masks and plastic suits slumped against the wall. He made the place come alive by telling it as a haunting “what if world war three had broken out” story.

Other communist-era relics had proven less robust. Next door to the Endspurt hotel lay an abandoned block. “DDR ruins,” said the owner, before serving me a cosy dinner by the fire in the Endspurt’s restaurant. I later nodded off to dreams of following an ever-lengthening trail of Rs.

I left the hotel in a blizzard the next morning, but the firs of the dense Thuringian forest soon sheltered me from the wind’s angry blast and, having stopped at many cosy cafes on the way, I picnicked in one of a string of south-facing mountain huts when the sun did finally appear.

By the time I arrived at the Vergissmeinnicht hotel in Oberhof, I wished I could go on to complete the trail, despite the bitter wind. I had covered just 60km in three days, and yearned to go on to historic Eisenach in the north. I could also retrace my tracks to Blankenstein in the south, once on the other side of the Iron Curtain, completing a journey that was never possible during the Cold War. But it was time for this spy to come in from the cold.

• The trip was provided by Thuringia Tourism (thuringia-tourism.com) and Hiking and Biking in Thuringia (rennsteig-und-mehr.net). Four nights’ B&B with luggage transfers costs from €358pp. For trail conditions see thueringer-wald.com. Ski hire at Sport Service Marr (sportmarr.de) costs €10 a day. Lufthansa flies to Frankfurt from London, Manchester and Edinburgh from £104 return, and has Rail&Fly train tickets to any German station for €58 return. For further information visit germany.travel