Mountain biking is perfectly suited to Scotland's rugged terrain and in summer time, some of the ski resorts transform into down hill mountain biking centres. One of the most prominent places that does this is at the Nevis Range which includes the UK's tallest mountain.
One trend thats gaining popularity is fat bikes. These are downhill bikes with very large wheels and the added weight helps build momentum on downhill tracks. One great advantage of going to somewhere like the Nevis Range is there is a gondola at the ski slope which takes some of the effort out of mountain biking.
One useful resource for mountain biking is provided by Developing Mountain Biking which offers a very useful interactive map plus a mobile app. It is also possible to go on a bike tour and this can be a great way to get fit and a great way to meet likeminded people.