I agree about the TFTNA book, lots of good info in there on building up endurance. Hiking with a heart rate monitor at first for me was key to keeping an eye things and optimize your time on trail for training.
I was pretty much in the same boat as 2600 in the middle of 2015. Late 30's and starting to do more hiking with my son as he had recently bridged to boy scouts. I thought I was in pretty good shape until I hiked up to White Mountain Peak on an open gate day with a friend. I had old fogies and little girls passing me by. It was an eye opener. At that point I started hiking in the foothills of the San Gabriels a couple of days a week and taking advantage of the gym at work. Since then I've done a lot more trips, dropped 40 lbs, and can do high elevation gain hikes and not feel wasted the next day. Longest single day so far was a loop in the San Gorgonio Wilderness for 24 miles and 7500' of elevation gain/loss.
