your track coach is not gonna like cross-training. A couple guys on my track team were gonna do a triathalon, and asked the coach what he thought of cross-training, and they were almost kicked off the team.

if you are a decent track runner, you are there to help the team, you are not helping the team if you are there to just get in shape. maybe if you are actually "slow" the coach won't care, but I wouldn't bet on it.