after owning an incredibility active, and bright working dog cross (mines a Chesapeake cross). I think its worth emphasising the incredibility bit.
These types of dogs are meant to have constant company and be given lots of things to do. So unless you can commit 3 hours + a day to exercising and training them, you are going to be replacing a lot of chewed up broken furniture and are going to get very very pissed off with the dog. They have been selectively bred to be bright and tireless workers, and that energy has to be directed somewhere.
Mine for example, until he was 3 would happily empty the contents of several wardrobes into my hall and chewed off several dado rails off entire rooms when he was left at home alone for a few hours a day and this destructive behaviour was after having a minimuim of 3 hours worth of walks a day (running, walking or chasing a bike) and lots and lots of training.
Just to warn you.
@z1ppy - yeah, aware that they need a lot of attention, somewhat like myself! Hence the choice. Would be living in a house with an ultra runner and a MTBer who errs towards the longer side of riding.
The running will be fine, the long long rides however may kill the dogs hips plus you will have to be giving it lots and lots of water. As whilst the dog can go all day, it cannot sprint all day, which when following a bike it could be running much faster than its all day pace.
Mine would run all day following a bike, but I wouldn't dream of making him follow me for more than 3 1/2 hours as the dog will kill itself trying to keep up, and I wouldn't want to hurt him.