I am a 47 yr old, 10 yrs + running, none competitive, until last year when I decided to run my first marathon in NYC. I got tendonitis about 6 weeks before the race, I blamed it to doing or over-doing yoga. Luckily I was able to recover with physical and massage therapy, and finished the marathon in 4 hrs 8 minutes, at a nice cautious pace and no pain. I entered Chicago this year, and have completed base training with no problems. Taking into consideration my age and history, how do I minimize the risk of injury while trying to get the 3:50 necessary to qualify for Boston?

Get a heart rate monitor and follow the guidance given in some of the links in the low HR training post in the "running articles" forum. Run a good amount of mileage, at least 50 mpw, but all low intensity.