As you already know, hamstrings can take seemingly forever to heal because they get well enough to run and then reoccur. I write about hamstring rehab in tips 38 and 54 in my book 200 Tips Every Runner Should Know, but here is the brief version.
Since you could handle speed earlier in the week, I'd guess that your warmup wasn't enough to loosen the hamstring OR you slightly reinjured the hamstring in your earlier workout or during the warmup. Then, it flared up in the race itself.
Since this is very early, avoid stretching, especially static stretching. When you can walk without pain for a full day, you can return to slow jogging or spinning the next day. Scar tissue takes 2 weeks to begin to coalesce and 5-8 weeks to get as strong as the surrounding muscle tissue, so you need to be especially cautious until at least late June.
Once you return to running, you must avoid speed, accelerations, hills, sprints, and hard push-offs for the next month or so while the scar tissue is getting stronger. Most runners return too soon, re-tear the weak scar tissue, and healing OF THAT STRAND OF TISSUE must start over from day one.
After a couple of weeks, you can add dynamic drills (scoops) after you've jogged a bit to warm up. Do them only to the point of tightness and quickly release. Don't hold the stretch. The scar tissue is still weaker than surrounding tissue and you can re-tear it by simply stretching too hard. Stretching harder is NOT better.
After about a month, you can conservatively add back some speed and keep your workouts much shorter at first. (I might do 3 reps instead of 9-10 on my first time back and I'd do those slower than my training plan called for.) If I felt no pain, I'd do 6 reps the next time and return to my full schedule after that. If you feel a pain, it's too late. The scar tissue has re-torn and that part of the injury has to start healing all over from day one. This is what most runners don't understand.
Finally, once you're able to start running normally, you can add some drills to maintain range of motion in the hamstring. Before a track workout, I ALWAYS do an easy jog followed by lunges, A skips, butt kicks, karaoke, high knees, adductor drills (I don't know what they're called) and for hamstrings: scoops, and leg swings. I didn't have to do these drills when I was younger.