Not silly at all. But very difficult for us to answer for you. When I was young, I would regularly hike 12+ miles a day with a 45+ pound pack, and often much farther than that. These days (now 60) that seems like a lot. The factors that will affect your mileage totals are things like the steepness of the terrain, the weight of your pack, your age and physical condition, and your goals for the hike.
If you are young, in shape, and focused on a through-hike of the JMT, you might be able to do it in one go--figure about 12 days and 17+ miles a day.
But I would never do it that way. I'd take 15-20 days, and re-supply half-way along, which is what most people do. That allows me to keep my pack at a reasonable weight, carrying food for only eight-ten days. And it also allows me to occasionally look up and see the scenery, and maybe even catch a few fish.
And your mileage may...no, certainly will vary.