Hi Beth,
It's such a difficult thing to predict. Many new runners cannot run but use a walk/run technique to build up to a 30 minute jog. I started in May having never run before but with a good cardio-vascular fitness. I ran then (and still do) 10 min miles on road, 11 on cross country. However over that time I've improved my endurance. I would have no qualms entering a half marathon and run 8 miles regularly.
Its a personal thing, Beth. Look at your ability all round and decide what is important to you, and set personal goals to achieve it.
I would love to run a 9 min mile, but also be able to run and complete a marathon. They are my personal goals.