45. Been at a desk job since 18. Did 400 miles of the AT then starting at 8 miles a day and working up to 16 (with a very heavy pack). I quit the thru bike but returned for visits and did a couple 20+ mile days a few months later. Years pass and got down to where a anything more than a couple miles hurt. Moved to a city with a strong outdoor walking and biking culture a few months ago and on day 1 the 4 flights of stairs to our flat killed me. I now think nothing of walking 10-15km or biking 30 in just an average day moving about.

Gradual increases seems to be the key