I live in Marquette too, and can offer some insights.
The south marquette trails are really good and close to town, if you're staying at a local hotel. Expect anywhere from 600 to 1000' of climbing per loop out there, depending on how far up each trail you go. There are typically loops that cut off a little climbing, especially over at Marquette Mountain. They've been expanding the trail system there, some are more xc bike friendly, although you're want to be careful with some trails.
Harlow is more low-key, smoother trails. Not as much climbing to be had, although the Top of the World climb is pretty decent going up. It's pretty fun to come down too, just watch out for climbers. The backside (Jedi Trail) is much fun too. The bareback trail is pretty rocky, with some hike-a-bike sections (unless you're pretty trials-ish). You can actually ride to harlow on the rail trail from town, which adds another 20 or so miles to the ride.
There's a little singletrack to be had around the start of the Noque trail, by the tourist park. It's pretty rocky and not ridden as much as everything else around here, but it's there.
There's a wednesday night ride that starts at the brewery in Ishpeming, which is typically a little faster paced ride, and it is NOT a NO-DROP ride, so be prepared to keep up. Good fast trails over there, speedy two-track and fast single track stuff.
If you've got the time, Houghton (2 hrs. away) has a lot to offer, but if you're going that far, I'd suggest going to Copper Harbor, which is about 3.5 hours from Marquette. Lots of trails, kinda XC-All Mountain oriented. Everything is accessible from town too, and multiple loops are there, are typically well mapped and you're likely to be the only ones out there.
Lakeshore bike would probably be the best place to start for trail help or try to hook up with someone local. I'd offer up my services as a guide, but my daughter (1yrs old) has eaten up most off-road time. Any saddle time now typically involves a chariot.
Feel free to PM me if you need some more info. You probably won't get lost, as all trails pop out at some trailhead. It may not be the one you're aiming for, but you won't die in the woods looking for the way out.
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