I did the Gotthard pass in Switzerland last summer. Physically, it was the most strenuous tour I have ever done. I was not in the best of shape when I began my tour, and I hit the pass on Day 4. It took me one-and-a-half days to climb to the summit, and by the end of the second day, I could not deliver power to my legs. I pedaled for two minutes and then had to rest five minutes. The altitude gain may have had something to do with the exhaustion I experienced. I needed six days of rest (no bicycling at all) before I was able to continue my tour. I avoided intense climbing for the rest of the trip, although I did do several one and two hour climbs without significant problems.
If I were doing that kind of tour again, I would do some things differently:
* I would not attempt to climb mountains without prior training, or at least without having spent many days riding myself into better condition.
* I would take a day after an arduous day of climbing to sleep, stretch, eat, and hike/walk.
* I would consider installing an even smaller granny gear than I already have (my lowest now is 24/32).
There may also be an age factor at play. I was 48-years-old last summer. My climb up Goddhart would probably have been easier for me twenty years ago. On the other hand, I now have bragging rights that I climbed the Swiss Alps on a bicycle 18 months before my 50th birthday!