I shot XC last season.
Don't bother with a zoom. Blow your money on a prime, because you'll need it. You'll probably be shooting the runners as they approach you, so if you don't like the framing just wait a couple of seconds and it will change! Even when I tried a zoom I found I usually didn't have time to adjust it once the runners started coming.
The key is getting fast AF. Fortunately, it appears you already have a K-5 II, which is the best you can do. Here's what I use (with one lens on my K-5 and one on the IIs):
FA*85
FA135
F*300
The DA*50-135 is an excellent lens, but it just doesn't focus fast enough for sports - especially XC. The FA (and also the F, I presume) 135 works really well. I think the DA*300 is slightly slower than the F or FA* 300, but according to other sports photographers here it's fast enough. If you
really want a zoom the latest model Sigma 70-200mm f/2.8 (EX DG APO OS HSM for Pentax) is the only one that repeatedly gets recommended for fast AF, but I don't like the IQ as much.
You don't want to go any wider than ~70mm. I tried the FA43 and I couldn't get any good shots because the runners had to be too close (and were literally past me a second later). I sold the FA77 before I got the FA*85, and I've had trouble getting good information on whether it or the DA70 focus quickly enough, but they might. Also, the FA*200 will likely do well, but absolutely NOT the DA*200 (I tried it - a nice lens, but for some reason way slower in AF than the DA*300).
If you were already thinking of buying any of these primes now's the time. The FA135 is the cheapest, and its wonderfully small size and IF means no one pays any attention to you in any setting. Also, order a BG now since you'll probably be doing 80%+ portrait, and your arms will get more tired otherwise (after the first meet I immediately plunked down $175!) - even this one is good enough if you don't need WR (I got it for my second body):
Battery Grip For Pentax K7 K-7 DSRL D-BG4 - Newegg.com
Here are some shots from last year. You can also see the basic EXIF information if you need:
Highlights - Orange County Championships 2012 - Digital Studio - RaceTime Photos Highlights - CIF Finals 2012 - Digital Studio - RaceTime Photos
Naturally you'll want to position yourself on the course according to the lighting you want. Sometimes only a few spots on the course work well (and these spots may change as the day moves on). I've got a lot of room to improve, but already I like my shots better than 75% of the photographers at these meets. Get at least one good lens (which you'll use after the season anyway), open the lens fairly wide, take some care in shooting and you'll have a few shots you (and others) really like by the end of the season.
Last edited by DSims; 08-28-2013 at 02:48 PM.