I wanted to see what HRE would have to say... "Thou shall not tempt the Lord thy God!"
You remind me exactly like myself 20 years ago (sorry to say...). I was up around 90~100 miles a week and feeling pretty strong. Did my runs around 6/mile pace, had to slow down to 6:40 pace for my weekend long runs... But my times weren't necessarily coming down (1500 and 5000). Upon studying my program, what Lydiard told me was "Do more repetitions." Also when I asked John Robinson, one of Arthur's Boys, he got his "Running with Lydiard" book out and showed me the schedule and said, "You've done this part (conditioning); now you need to move on to the next phase..."
You seem to have done plenty of "base" work now. If you want to peak, say, the end of June, you have about 12~14 weeks now. That means you should be doing Lydiard's Hill Training NOW. What makes you think you need to "back off"? And what makes you think, if you work on quality, you would "hinder your future development"? Quality workouts don't hinder your future development; doing them at wrong intensity, wrong volume, at the wrong time MIGHT. In fact, with almost-4-minute-mile background and your hope to make Olympic Trial standard in 5000m, you might want to consider sharpening up more for the mile first. I said this before so just to make clear; I don't necessarily believe in running shourter races for longer races all the time. I didn't buy the idea of Stember running 800 for a season or two for 1500m in the future. On the contrary, I thought he should have tried 5000. But I noticed in more cases than not, people like to try 5/10k simply because they are more convenient (more opportunity including road races) and they tend to neglect speed (again to make things clear; when I say "speed" on behalf of Lydiard school, it's "sprinting speed" not necessarily "anaerobic speed". You can work on it without having to go through classic intervals such as 12X400).
A couple of suggestions, moving forward; your total mileage sounds very solid but, when you take a look at your program and considering your training speed (6:30 pace), you are "only" running a little over an hour during the week. For 5000m, I would try to push the weekend run to closer to 2-hours and at least one more around 1:30 during the mid-week. Your comment about "nagging pain" worries me a bit and am tempted to tell you to slow down a bit. At least for those longer runs, try to stay closer to 7-minute-pace??? You seem to mix plenty of hills but, if you can, run on trials or cross country courses. All those running on the road beat up your legs. Running on uneven terrain not only rejuvenate (spelling?) your legs, but also strengthen otherwise underdeveloped muscles in your legs.
Moving forward for 2008 season, try 2-cycle program. This means you need to move forward with "other" parts of the Lydiard program (as described by HRE) and, after the summer racing season, moving on to another build-up phase. Pick some good cross country races or road races in the fall and train for them. I would say, for a 5000 man, work your way up to a half marathon distance. Depending on the timing, try a few indoor races as well (a mile or as short as 800m). Then move on to the next and final (for 2008 season) cycle. This way, you'll have 2 cycles under your belt before you try the "final" cycle. By then, you should know the program and know yourself. You should lay out a weekly chart leading up to 2008 outdoor season and fill in what you should be doing for each week (build-up, hills, reps, etc.). I try to have this made for about 1.5~2 year period for my athletes so we'll know what we'll be doing throughout the season. Don't just stay at the conditioning phase. All the other aspect of training are all "conditioning" your body. You need EVERYTHING. You don't eat a cake half-cooked.
For hill training, seek some advice from Glen McCarthy or Hotlanta Master. They know what they're talking about. HRe..., well, you'll have to be careful what he means by "skipping"! ;o) Just kidding!