I'd definitely go for slick tyres for riding on roads. Knobblies are great for proper off-roading but are hard work on tarmac. Okay, you want the exercise, but there is exercise which is a slog and exercise which is fun and I know which I prefer!
You don't really need huge tyres for road use either. When you think that most roadies ride 23 mm or 25 mm tyres (an inch or less) you can see that you don't need 2 inch tyres on this bike. I'd go for something no more than (say) 1.25 inches wide and probably with some sort of puncture resistant belt in. Something like this. I'm not recommending that particular tyre (I haven't researched it), but it's the kind of thing you should take a look at.
I wouldn't buy the cheapest tyres in the shop either - they tend to be heavy and the wheel rim is the worst place to have excess weight on your bike.
Cheers for that crc are a good site too I've had lots of stuff from them, My Mtb had a accident last year at Dalby forest and I replaced the rear wheel with a secondhand one which is made od steel and I hate it , its hard work to ride now. I had been looking at the tyre weights. cant wait to pick it up this weekend and get it ready!