Protein Shakes for 40/50 mile per week runner?

I run about 40 - 50 miles a week and do a set of press ups (30/40) every day but no weights or any other type of exercise. I mainly want to improve my running times (5k 10K and HM) but I'd also like to maintain a bit more muscle mass on my upper body (for vanity's sake). I find that all the running I do is great for getting rid of body fat but I think I've also lost a fair bit of muscle mass. I eat an average diet - cereal for breakfast, sandwich for lunch, meat potatoes and veggies for dinner with a couple of bananas or nuts for snacks (and a LOT of coffee), and occasionally some junk food like crisps, chocolate etc. My weight is currently stable (BMI 20.9) so I think I'm getting enough calories. Would protein shakes after long runs improve my performance in races and would it help me retain muscle mass on upper body?

Thanks for any advice.

After a long run I think the focus should be on recovery, to get the glycogen replenished, and to aid muscle recovery/repair.  I think the ratio cited is something like 4:1 or 3:1 carbs to protein. I eat normal food for this; many use recovery shakes / drinks. Getting a quick recovery is the aim so you can give you next hard session 100%. Maybe a protein shake is better later in the week, or increase the protein you eat daily.

If you are not finding it too hard to do the 30-40 press-ups, you might try gradually increasing that each day, either by doing two sets or by doing a few more each week. You could also get a pull-up bar.