After 8 months out I've just done 4 weeks at 18, 30, 30, 30 miles jogging 8:30-9 min/miles. This morning did 4 miles at 6:40 around the block as some sort of tempo effort.. Last autumn I was running 17:50-18 for 5k and my tempos were 5 miles at 6:30... so this isn't too bad but I can tell I have lost a lot of fitness.
Instead of focusing on miles should I try doing lots of tempos? Or stick with lots of hilly moderately-long runs (this is what I usually do).
Focus on the mileage or time spent running at first. You can do strides, some short reps at tempo pace, and runs where you finish with the last couple miles hard to touch on speed. The most important thing coming back from long term injury is to build a base to prevent recurrence or another injury.
I agree with Kvothe but I would add that one way to help expedite your return to fitness is to double up on workouts a few days a week. One thing that helped me after weather limited my winter training was to do a short speed / strength workout in either the afternoon or morning in concert with a longer easy run to build an aerobic base.
These don't have to be tempo runs or even track drills but anything that's going to help build your speed / explosiveness think:
15 to 20 minutes of hill workouts
a 15 to 20 minute tempo run
Box jumps or plyometrics
Squats
Doing 2-3 of the above per week helped me get back my fitness pretty quickly.
Lots of doubles. Everyday except long run day. 4-5 miles each session with 4-8x100m strides twice a week. Do some steady and tempo runs but never to hard. The key is to get in a lot of easy volume and some moderate efforts while recovering as fast as possible. Ripping intervals will leave you too tired to train/recover and improve. You’ll get in shape fast but this is not a good strategy to get faster.