I want to train myself until I get a personal trainer in May

So I want to run a 49 or 48 by September/October. I ran a 53.6 in the 400m back when I was 15 and once my coach left, I never really trained since I wasn’t really motivated. I’m currently 19 years old in college and have access to a indoor track and was just wondering what exercises I should do to train for the 400m and 200m before I get my personal trainer in May.

Thanks guys!