One of my buddies claimed to have run PRs in the 800 and 1500 off of just long runs, tempos, and a weekly race or time trial. I find that hard to believe, so does that even seem plausible if you've been running consistently and your PRs in those distances aren't horrendous?
FWIW, his times were 2:05 / 4:12, and he claimed that he always fell apart in the second half of the race whenever he included intervals and weights in training.