Hate to be a wet blanket, but your math is waay off!! you plan to ride approx.120 mi. each week to ,work(taking into account bad weather days),then ride 100 mi each way to the dirt(200 mi. total).If you use a 40/60 ratio you would be riding in the dirt approx.360 mi. a week!You would need tires that really aren't suited for someone new.
I would suggest a 80/20 ratio.You could then use a tire better suited for you.Single track should be a year away,however, there are plenty of dirt/gravel/poorly paved roads to keep you busy untill then.Get on the country roads and go riding when you see a dirt road: take it!!
My wife is 5'9" and she 49 Y.O. She got a DR 650 and we did just as I have outlined and she has become a good rider.
She was exactly like you,(wanting to run before she walked)Hell, we ALL are like that or we would be driving cars!!.
After her DR650 took a couple "dirt naps" she realized there might be a little more to riding dirt than she thought.She had been riding for 5 years(Gpz550,Vulcan,1975 goldwing,etc.All bikes I had sitting around) before this.
The DR 650 has a Factory built in rear shock lowering system and you can lower the forks a little in the triple tree.It's simple to work on and you can't kill it.
If speed is a concern, a friend took my wife's DR650 out for a spin after we had done some work on it and came back with a GPS reading of 105 MPH and he weighs 145 lbs.
As far as being able to put you feet on the ground,take the bike and put the front wheel on the curb(like you might find it on rock while single tracking and then decide how much of your foot needs to be on the ground).
I'll post a pic of one of her "dirt naps" later.
Sorry for being so long winded,but,I want to enjoy riding and slow is fast ...