You pose a difficult question because we aren't sure whether you want to return to the fitness levels of a soccer player or the fitness levels of a marathon runner.
George's advice to cross train on a stationary bike is great. You might be surprised how a fit runner like yourself won't be competitive initially against the other spinners in your gym! Biking takes slightly different muscles than running. Try the recumbent bikes (my favorites) that will give the most stability to your shoulder.
Jeff Galloway says you can measure your fitness retention by the length of your last long run. He says you will retain your aerobic fitness for 4 weeks after a 26 mile run and for 3 weeks after a 20 mile run. Go to the jeffgalloway.com site and read his archived "Back Pain, Part 1, Vol. 9, Sept. 1999".
As a soccer player, you may be as interested in anaerobic fitness as in aerobic fitness. Here is where we may part company. As an older marathoner, I'll do a fast 10k for speed work. As a young soccer player, you are probably more likely to do multiple 440's. The archives at RunningTimes.com will give you several Q&A's about 300-yard, 440-yard, 1-mile, and 2-mile training programs.
The typical advice that I see for injured runners is to come back slowly. Concentrate on the LSD aerobic runs before pushing yourself on speed work. Don't risk injuring your legs when recovering from your shoulder injury.
Lastly, I sympathize with your plight. As a youth soccer coach, I read that book -- probably now out of print -- by the journalist who tried out for the defunct Washington Warthog semipro soccer team. He described himself as being in excellent physical condition, but he couldn't keep up with the second-tier soccer players trying out for the Warthogs. Competitive soccer is all about running fast, stopping, turning, running fast, etc., for 90 minutes, while under pressure from opponents.
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