I am a early-30s woman and started running in March. I love it. I especially like the longer, slow runs. (For me longer is 8 miles). Can anyone weigh in on whether there are physiological disadvantages to doing just "long" runs? (Admittedly, they cease to become "long runs" if you don't have short runs, of course.) If I wanted to run 8 miles three times a week, is that worse for me than running 4 miles four times a week with one 8-miler? (Assuming I have built up to that weekly mileage). I know my speed will not improve the same way as if I vary the run distance and pace, but if I am not concerned about speed. I just want to enjoy my running, and I really prefer the long ones!
From what I've read (both on the Cool Running website and in Jeff Galloway's books "Galloway's Book on Running", 2nd ed., "Running Until You're 100" and "Training Programs"), you don't HAVE to do short runs if your body recovers fully doing what you're doing. You're right that offering your body different types of stress -- a slow long run to push back your endurance wall, a faster, shorter run to build strength, hill training to build strength, speed intervals to increase pace, etc., etc. -- would help diversify the benefits you get physically, but it seems that there are lots of folks out there whose "daily" runs are longer than 3-4 miles and who don't bother with any of the other stuff. The philosophy of running that I've chosen to adopt is that it should first and foremost be enjoyable, which will help ensure that I keep doing it, which will mean that I keep getting all the physiological benefits from it!
You might want to pick up Galloway's Book on Running. It's got a lot of info and has A LOT of stories about runners other than him. The Running til you're 100 book also shares stories about how older bodies take longer to recover and how to adjust running to meet those needs ("older" is generally >40, so you don't have to worry about that!). But one story that might be applicable to your situation is a guy in his 50s who used to run 5 miles every day (or maybe 5 days a week with one additional longer run) who discovered that as he approached 60 that he had aches and pains that didn't go away between runs, and so he combined his mileage into easy 10-mile runs two or three days a week and actually felt better because he had the full day of rest between. So, I think you're in good company, and that unless you develop aches and pains that could be relieved by mixing it up a bit or unless you stop enjoying the long runs, then you're doing just fine!