How fast can I get at marathons? I'm female aged 48.

I'm guessing if you work your butt off, and get your 5K under 25 minutes in the process, you can break 4 hours. Your Holy Grail would be obtaining a Boston Marathon Qualifier.

Short duration track work with lots of rest days. You actually have been working too hard without enough rest. Once a week run a few 800's with rest between intervals.

You already have raced and run so much your muscles may be beyond the point ofrecovery.

Forget about "fitness classes"; to get better at running, just run. How many miles a week are you doing? If you're putting in decent mileage, are not overweight and only able to run a five hour marathon then something strange is going on. A 5 hour marathoner doesn't need speed work - you need better aerobic conditioning.

800 repeats? That's not short duration.

If you want to run fast over distance you have to be faster at sprints. Bekele would never have ran 12:37 if he didn't have the form to run a 3:33 1500, or something like that. And no doubt he could have ran well under 1:50 for 800.

Your form, how you run, is the "baseline." Then you figure out how to maintain that over distance. If you approach that backwards, sticking to a 100-mile form and trying to figure out how to make it faster, you're stuck in the envelope of that specific form and limited to results well below what a better form would achieve.

Nothing over 200m is "speed work" for intervals. 300m or 400m is speed only if recovery is not time-limited, and 600m is speed only as a time trial. To work your speed, do not hamstring your efforts with fatigue.