First, 2082 had incredible advice. Follow all of it.
I self taught how to swim, but I still regret it. I'm only ok at it because my body type is a near match of Phelps, and I study videos like I just broke from the Matrix. I'd obviously favor getting a coach if you have the time/money.
Overtrain the lengths of your race just like 2082 said, especially for a sprint. There is no reason not to at those distances.
Also, you should have at least one recovery day and two to three days of lifting as well. Your body will degrade if it's only cardio. The lifting doesn't have to be long if you are smart. 20-30 minutes of low rep strength work is plenty of you are efficient.
You could never stretch too much. You are probably working on your mobility half as much as you should at best. If you keep doing these, it will catch up to you really, really fast if you aren't working on this as well.
Food? My biggest recommendation and bummer: No alcohol for at least a month prior to the race. Zero. If you can expand it further than that, you will be even happier with your result. I'd also eliminate all dairy in that time frame. If that feels too restrictive, I'd only allow great fermented dairy from reputable brands like Siggis. Why? No cramps. Ever. I'm totally serious. In the times I eliminated alcohol and dairy, I never once had even the hint of a cramp.
Other food favorites: sweet potato, berries, zero carb isolate protein powder (like isopure), chicken and rice.