You've got good advise above Paul, can't add much there. On a more general note though, I do like the shorter faster races myself in some ways. But my opinion is changing. Last year I was doing speed workouts every week or two, plus pushing speed a little more in general. Striving to get my 5k time back in the range I used to run in HS. And for the first time in the few years I've been running as an adult, I did. Now my record HS 5k is still nearly 2 minutes faster but 19:28 atleast got me in the mix. Some dismiss 5k's as easy, but I tell you that was harder than probably any other race I did last year even including my first marathon. Granted it was over a lot sooner and recovery was quicker. But being so far out of breath, moving quick enough I could not even see obsticles on the trail (luckily they cleaned the trail surface prior to race). Tiny inclines I never even notice was there seemed to turn to huge hills that never ended lol. In contrast, the marathon I did a few weeks later was such an easier pace - in fact it was the first real 'race' I was not struggling with breath at all, and was practically comfortable (though that might be a tiny bit of a stretch). In fact when I hit 20 miles I was surprised how much energy it felt like I had, compared to the turn around point half way through the 5k where I thought OMG I'm going to die.
And the 5k hurt too. Well I think it was the 5k but not sure... I felt a little sore after but no big deal, I kept on my usual training without any taper. The following weekend I was going to do my longest run yet, and 1/4 the way through I had unexplainable leg pains. It kept getting worse, and I had to bail for the first time ever mid-run and call for a pickup. I highly suspect the 5k race (and thinking afterwards that I didnt have to recover) was to blame.
I think it feels good to go a bit on the fast side, and I think my form is better fast... to a point. But I think at my 5k race pace I'm going beyond what I can hold as good form and by the end of the race I'm slamming my feet pretty hard. So those shorter faster races have there own challenges. If I went a little slower it probably wouldnt be an issue, but I just seem to have the drive and energy to always push harder than I probably should.