Let me say that you could run a half Saturday if there was one in the neighborhood. If you are looking for a challenge, then step up, though it needs to be something you are driven to do as November and December can get long.
I think your speed outdoors will vary some from the mill, but not all that much. You will not have the motor effect so it will feel a little harder, but in all likelihood, your stride will lengthen a bit creating a little speed.
I would only use your TM speed as an estimate for where you start. You do not say where you are exertion-wise on the 7 mile run... if you are only at a 6-7 level (out of 10) and could carry on a nearly normal conversation, then you are at a great training pace. If your 7:30 pace is a struggle and you are huffy puffy; only able to say a couple words per breath, then you need to slow up.
I have a little different take on submitting a time. Submit a 5:01 expected finish time. It keeps you out of the last corral and does not need a back up race as proof of time. By doing this, you take the need for a fall race off the books. Though at a 7:30 - 8:00 pace, you will want to get that fall race in. You will want to start in the proper corral to allow for a good race.
Like Maura, I only run 3x a week, sometimes 4. But I am also in my mid 50's. With your base, I would look at Higdon's marathon plan. I think he has you out 4-5x a week. You need to follow a plan for your first marathon or so. Most plans include a periodization build; or saying it another way, they increase miles 2-4 weeks, then fall back for a week as a recovery week. It is important to allow for the recovery weeks as you build miles. These are actually the weeks you gain strength.
Hope this helps.