Get a hardtail.
For 600 to 700 quid look at Giant, Decathlon, Merida or the Boardman range.
The big three of Trek, Cannondale and Spesh don't offer the best spec at this price point, neither do Scott, all the mid range Scotts I have worked on have been resoundingly crap in build quality and spec for the money.
Don't buy a Trek, their mid range hardtails are not very good at all, they look awfull and they are very heavy. Kona offer poor spec at this price point as well, they are and have been over priced for a very long time.
Out of interest One-One are doing a carbon race stick for 1000 at the moment if you are prepared to put up with "quirky" customer service.
On the fitness angle.
A lot audax types struggle with MTB riding as you need upper body strength to get the most out of mountain biking, audax tends make your upper body atrophy. Upper body strength and a bit of muscle also stops you breaking things when you fall off or hit stuff.
Being skeletal is bad for you.