In reply to Jamie Bankhead:
If you want a good mountain bike day out, cycle in from Glen Tanar, cycle/push/carry to the top of Mount Keen from the head of the glen - it's steep, but short. Then cycle down into Glen Mark down the path on the other side of the hill - it's a fantastic descent. Then, cycle down the glen for a few miles and head up and over the Fungle Road (a path, but great on a mountain bike), which takes you back to the start. It's a fantastic day out on the bike.
If you are doing Mount Keen from the south though, it's not a big day just walking it. Done it easily in 4 hrs return without trying too hard. However, you are more likely to be able to cycle most of the way to the top from the Glen Mark side.