I had the diagnosis of degenerative meniscus tear last year and had surgery. After about 4 months, things were going good. I was up to about 12 miles per week, with some speed work thrown in and was given clearance to "go at it." I was told I had at least 5 more years in me. Early this year, the knee started going wonky. I restricted my run to one 3 mile run per week and 2 3-mile walks. I did a sprint last week and did an 8:20 pace for the run. I was pretty happy with that with what little run training I was doing. However, it hurt for a few days afterwards to go up and down the stairs. I'm going to focus on swimming and biking and strength training and don't plan to run any more this summer. :-( I want to be able to enjoy my knee for another 30 years. BTW I'm 60 and was a runner for 47 years. At least I can say I had 47 good years of running (started in high school.) I'm going to focus on swimming because it's completely pain free. I signed up for a 5K open water swim in June. Pardon the pun, but I'm not dead in the water. After that I plan to do aqua-bikes....there are a lot of them out there. I will give running a shot next year and see how it goes. In the meantime, I will continue to walk a good bit. I may also do the elliptical at the gym.

I'm 60. This is what works for me: Usually 3 runs a week. Two shorter runs, at 5 miles each. One long run at 7-8 miles. So total mileage is 17-18 mpw. Most of the runs are on trails. ALL of the training runs are in the 9-10 min/mile pace (so much slower than your training run paces). My ONLY hard runs are in races, and I try to run at least 1 race (running or tri) per month. My race paces are similar to your's: 6:45s for 5ks, 7:00-7:05s for 5k at the end of sprint tris.
Summary--most of the runs are easy and aerobic, and the low mileage doesn't beat me up. Only hard running is in racing; races are as fast as I can muster.
Of course, I also bike 3 times a week, and swim 3 times a week (and the swims are pretty intense), so aerobically I'm getting plenty of work. I just don't try to run too much or too fast (except in races).
Hope you find a formula that works for you.

I did not read all of the comments on this thread, but can you try this:

1. At the end of every swim add 15 min of vigorous water running
2. 3x20 min per week jogging on the local socccer field on grass with some pick ups
3. 1x20 min per week treadmill hill intervals fairly steep at low speed assuming your calves and achilles can take it (build up...you'll be surprised how useful running 5 mph at 8%-15% grade ends up being)...pounding is limited. This is a bit more specific than elliptical to real running
4. Never run on pavement except race day.

The above should more than cover you from sprint as you are an accomplished life long runner and KNOW how to run. Your brain won't forget how to run, your body just has to follow and it has to be intact to follow.