400/800 breakthrough

Definitely possible, I basically did that last year. One important factor is: what was your training like last year? Training history? If you were poorly developed, both aerobically and in speed, then it would be much easier.

Breakthrough wrote:

Have PRs of 56s in the 400m and 2:10 in the 800m. Is 52 and 2:05 reasonable to get this year?

With a 56s 400m speed you should be able to do a sub 2.05 already, so you lack endurance. Work on the basics, aerobic endurance, sprinting 1-2 times per week (short fast hills with long rec are great), not overdoing it. Getting down from 56 to 52s @ 400 is much harder to do than going from 2.10 to 2.05 @ the 800.

My best 400m time is 55 seconds in a relay split because I've never done an open 400m but I think I could have easily gone 53-54 because of my hand off. My 800m time is 2:02.75 so I'm guessing based of your PRs you should have dipped under 2:05