You're basically describing my life from age 25 through now (35). 35mpw at a casual pace will keep you fit but you won't feel like you're in racing shape. I've found that even when I get motivated for 2 to 3 months to "train," my times only improve incrementally. For example, I can pretty much get off the couch now and run 6 min pace for a 10k. That said, if I were to up my mileage to 50 mpw with a workout, tempo and long run per week and prepare for an all out 10k, my pace would likely only improve 10 to 15 sec per mile. Granted, I'm not exactly training hard, but the point is that once you reach a certain point in your life, the time, motivation and rationale to train like a real runner do not exist. I would need a year of real training to produce any significan gains and what's the point? I'd rather spend time with the family, eat out, play golf, go to sporting events and sleep in when I feel like it, all things I might otherwise sacrifice to train like a real runner.