Well, it depends. But I would say its doable and still fun.
You'll drive about 3.5 hours to Waldshut-Tiengen where the fun begins. So 3.5 hours highway is not a big problem. I don't know about the break in, but it's mostly unrestricted highway.
Then you do a little break and then begins the fun. Through black forest you can go always back to the highway if you're fed up. If you do the full trip through black forest (4h) you end up with 2.5h highway left to the Nuerburgring.
Sounds ok for me, but still a long trip
@EmmDrei: Well, of course there will be more traffic but if you just have the Saturday there is no option. Or skip the trip, but it's an special experience. Just my 2 cents.