However, know this: your gate, form, mind, and muscles are used in a very different way on a treadmill versus running outside. Think about it: on a treadmill, you are running on a belt, so you are never really engaging those muscles that make you move forward. You are predetermining your pace, which will often times limit the runner. Outside, you will most-likely go faster. Mentally, staring at a wall, in a controlled climate is nothing like running outside or running a race. On the belt, your gate never really changes and your stabelizers that would activate while going around a turn, downhill, uphill – you would first have to mentally change it by pressing buttons on a treadmill.
My advice: GET OFF THE TREADMILL!!!!! It’s boring. It can cause knee problems. It won’t prepare you for a race. The ONLY reason I would suggest my clients run inside would be if it is unsafe to run outside. Rain, heat, wind, cold – those are not good reasons. And guess what, you cannot predict what the weather will be like on race day, so you need to learn to adapt.
Bonus: Running outside is freeing. It allows you to explore locations you probably wouldn’t otherwise see. And it’s easy to find a running buddy.