I'm about 90% over plantar fasciitis but I'm still wary of doing any speed work and hills. Right now, I'm just trying to run long slow stuff, about 65-70% max HR or about 115-120 BPM. I'm following the Maffetone program which fits in nicely to my current injury problem since it's emphasis is on running long, slow distance in order to teach your body to burn fat primarily instead of glycogen. In the recent past, prior to injury, I was doing a two hour run/day at a nice easy pace (10 min/mile or slower depending on how I felt) and racing a lot on the weekends, mostly 5Ks. In 2007, I ran 15 marathons, qualifying for Boston 13 times. My best times were: 3:26 marathon, 19:44 5K, 43:50 10K. I do no cross training and no speed work other than the weekend races. I havent raced in the past year due to the injury.
As we get older, I think we must reduce the speed work or risk injury and/or burnout. I probably raced too much but I'm almost back now. At present, I'm running 60 mins., twice a day, early in the AM and late in the evening, trying to put 12 hours between workouts. I run 7 days/week. Again, the pace is easy. I'll keep to that program for a few months until I regain my prior condition and then race again.
I'm glad to share whatever I know. There isnt much info available for us geezers. If you can list your email address here, I'll forward more info. and links.
P.S. I'm 65 and have been running since 1958