What worked/works for me is day in day out riding. In my ealy/mid twenties I would throw the kid in the bike seat and go fo 30-40 mile rides without noticing it. The pursuit of $$$$$ and resultant lack of exercise turned me into a fat ass that could hardly walk a mile without needing ibuprofen. Last summer, due to an implosion
in the pursuit of $$$$ I ended up working in Glacier NP. As a family we rode everywhere... bikes were our only form of transportation. I averaged 15-20 miles a day, everyday, for 3 months. Much of it uphill. The difference was amazing. There is a stretch of rode leading to our campgroung that was about a 8% grade finishing in about a 10-12% for about a block. In the beginning it was everything.... meaning almost passing out when I got to the top, I could to just to get to the top in the end I could sprint the entire distance in my biggest gear. (the bike creaked and groaned alot)'
I started the summer a pudgy, out of shape 265# or so fat arse. Three months later, while I had only lost about 30 pounds, I had legs like lance
and an unbelievable (to me) ability to keep going. Remember when you were a kid? Thats how I felt.
It's almost three months later now and unfortunately I let everything go. Stress... a different envinroment and just plain being lazy has led to the inevitable... I've got back on the bike finally but I'm a long way from where I was 3 months ago. So ultimately, and in my opinion, the trick to increasing one's stamina is to simply do it... ride the bike MORE, ride it harder, increase your distance. I think time on bike really makes the difference. Look at the touring cyclist... many average 100 miles a day loaded and I've yet to see a fat long distance biker.
Doug