Totally realistic. If I ran 42 on the way to a 70-minute 10-mile at
age 53, you can burn my butt at 45.
I was training 40 mpw and doing 3 miles total of all-out repeats once
a week (plus warmup/warmdown) in various formats: halfs, quarters,
miles.
This was the wrong way to train. I wasn't increasing
speed-with-endurance.
For that, I suggest you do everything at about 70% of MHR except your
speedwork, and just do mile repeats most weeks, with a few quarters
every third week for leg speed. Walk a full lap <slowly> between
repeats of all lengths.
If you're able to run high mileage (50+), you can do just about any
kind of speedwork you can handle without getting overtrained, and
it'll accomplish substantially the same effect. This is Phil
Maffetone's advice, and he's got the stats to prove it; e.g., Mark
Allen didn't begin winning the Hawaiian Ironman until he started
training with PM. Maffetone's book Training for Endurance is very
good.
George Beinhorn
www.fitnessintuition.com