I've never had the best body for running. I'm not tall (good) but too heavy (bad) and I have flat feet (bad). However, I was able to start running at age 50 and by 55 qualify to run in the Boston Marathon without injury. I ran six marathons and probably a dozen half marathons in that time, eventually at a pace in the top 20% of my age group, not that great (or even that good) but I finished my marathons and was in fantastic shape.

Here are my recommendations:

Shoes -- keep experimenting until you find pairs that don't cause aches and pains. Some shoes caused knee pain in me, but others didn't. Find two different brands that work for you and use both pairs alternating between them. Each manufacturer's shoes feel a bit different and alternating you will lower the chances of overuse injury.

Form -- learn to feel like you are hitting the ground very lightly with your feet. Don't pound, land on your heels, or over stride. Thinking of contacting the ground directly under your body with your foot fall will keep you from landing too far in front and will reduce the strain on your legs.

Mileage -- limit long runs (over 5 miles) to one per week and once they get over 12 miles to once every two weeks. For most of my training I kept mileage to under 40 miles per week. Plan on taking over six months to build up to being able to run a marathon even if you start out in good shape. Build mileage slowly.

Frequency -- take days off to allow your body to recover. Most of my weeks I ran MWF about 3.5 miles each day, one day at the track doing intervals or hill training, and one long run on the weekend.

Run/Walk -- my first marathon was one of my fastest. Although the conditions were ideal that day, I also credit using Galloway's run/walk method [1]. During a race or long run I would walk through the water stops and walk for say 45 seconds to 1 minute during that time. This really gives your legs a nice break and I was surprised at how fast my overall times ended up being using this method.

Lose Weight -- I will never be a good weight for distance running (even in college as a gymnast with very low body fat I weighed about 150). However, distance running helps one lose weight and losing weight helps one with running. Being heavy greatly increases the stress on your body.

Finally, I would take aspirin or Advil for aches when needed to help keep inflammation down. Warm up before running, but don't stretch before running.

[1] http://www.jeffgalloway.com/training/run-walk/