Finding your Running Speed

EJsMummy26

I'm planning to do a 5k in early March, I'm in week 3 of running. I've ran outside but mostly on a treadmill (we live in WI, so it's cold). When running inside, my legs want to go faster but I'm exhausted. Seems like it gets easier the longer I run to pick up speed. I'm doing a 5 min warm-up/cool down.

Any tips? How do I find my optimal speed? Is it just my endurance?!