I was out for basically 4 months this summer with a slow healing stress reaction in my foot (started doing a little bit of running during months 3 and 4, but very slow short jogs and the vast majority of my training was cross training).
I didn't come back super fit, but not as bad as I expected and I ended up running close to my 5k PR a couple of weeks ago, faster than last spring before my injury.
While injured, I did (would move around the days depending on preference):
M - 75-90 min easy on bike/elliptical. HR 135ish
T - Short hard speed session such as 12x1 min, 8x90 seconds, 16x45 seconds (with around equal rest)
W - 90 min easy on bike/elliptical.
T - 75-90 min steady on bike/elliptical. HR 150-160 (first 15-30 min easy)
F - Threshold session such as 3x15 min, 5x8 min, or some combo (1-2 min rest) - goal would be to get HR to 185/190
S - 60 min easy on bike/elliptical
S - 2 hours easy on bike/elliptical
In addition to the bike/elliptical stuff, I would swim or "aquajog" a few times a week. My heart rate did not go up very much.
I don't know if it's necessary or even beneficial to do this much. I seem to do well with volume and I am very much a 5k/10k specialist. If you are more of a miler, you might not need a 2 hour "long run". Also, my heart rate is a bit on the low end so you may want to adjust. Good luck! Cross training is absolutely awful but it does work