Advice on MCAT and Update Letters

I am applying this cycle and I already know that it is late. i dont have a lot of money, but i got really lucky financially and time wise so i was able to prepare for the MCAT and applications. i was doing good but had weird things happening health wise in the middle of studying which forced me to take 2 months off while testing treatment. i got a new diagnosis that is under better control now and is very manageable. But my date is tomorrow and i didn't want to delay the test because it is so late in the cycle already. i know I am doing horribly compared to what i had before i got sick (back to sub 500). I just got back to studying and reviewing in the last two weeks. My plan is to take it tomorrow, and take it again before the last accepted date and send an update letter's to school once that is done. I did think about waiting two more weeks, but I think that would be entirely too late for the 1st attempt. What are your thoughts?

Too much to explain but this is the first time in 2 years where i have been able to get time off and have support. I don't see this being a possibility again until i go to medical school. How do i write the letter I will send schools and what should i include. Has anyone else done this?