For a 2-leg du it is less common to do the run first, but WAY easier! Going from run to bike is NBD...the reverse is where bricks are SUPER important to train your legs to feel less jello-like.

As long as you can comfortably do a training run with 2/3 of the race distances you'll be just fine. So 3 miles followed by 10 on the bike will have you adequately trained to finish. For a faster race do some longer runs, rides, and speed work and you'll be good to go!

Have fun. I love dus. They're my most favorite sorts of races, since I'm a mediocre runner and cyclist. We Jack-of-all-trades types often do better than those who are super strong at one sport or the other, but relatively weak in one.