You can ride up fire roads and get up to the big road gap or visit scotland wall ride easily- which unfortunately doesn’t get you up high enough for the good bits of the nevis red, but does get you some good (and very trailbikable) dh. I think personally I’d stop at the wallride rather than going up the extra bit to the road gap though as you just waste the extra height on about 4 wee drops and a jump.
I would not personally push up higher tbh, it’s possible of course but just hard work. The DH and Nevis Red both use their height pretty carelessly, it’s not like a trailcentre where every foot is usually earned and paid back miserly, they’re designed for uplift. And I also personally wouldn’t bother with the lower Nevis Red. The 4X track makes for a comical ride if you don’t know what you’re doing, like me, I don’t think anything before or since has made me feel like quite such a tosser on the bike 😆
But there’s bags of good riding lower down- the waymarked 10 under the ben route is abject pish sadly but the witches trails are good, and you can build the good bits of the 10UTB lap into an extended witches lap. Also you can add in the blue crane descent, it’s very easy to find with strava etc.
There’s wee bits of trails all through the forest but it’s kind of hard to navigate that way, especially for a first time. Or you could do the wee loop out down the whw, back up the old military road and round cow hill loop out of the fort.
