Practice and remaining calm underwater seems to be the key to reducing your gas consumption. Most new divers consume lots of gas due to the unfamiliarity of being underwater, novice skills, and the ubiquitous overweighting.
After a while, you learn to control your buoyancy and get your weighting correct: enough to stay down with a nearly empty cylinder (circa 30 bar) and no more. You also learn that winter diving means thicker undersuits / wetsuits which are a lot more buoyant, so additional weight is needed to counteract it.
With more practice you'll consume a lot less gas as skills become far more simple and an awful lot less stress.
Gas consumption is measured as the "Surface Air Consumption" rate (SAC rate) in litres per minute.
As we all know, for every 10 metres of depth, add 1 bar of pressure. So, at 20 metres, you'll be at 3 bar which is 3 times the surface pressure. If you have an SAC of 15 litres/minute, you'll consume 3x15 = 45 litres per minute at 20 metres, or 60 litres/minute at 30 metres.
Your SAC will vary greatly depending on what you're doing underwater. If you're vigorously swimming against a current, your SAC will easily double and could quadruple in extremis. Those numbers again: if you're really fighting the current your SAC could be 3 times normal, so using 15 as the base, this will be 3 x 15 = 45 which is then multiplied by the pressure, so at 20 metres this is x3, so you're consuming 135 litres / minute, or converting that to bar of a single 12 litre cylinder, that's about 11 bar a minute -- dangerously emptying the cylinder in 13 minutes (using 12 litres x 150 bar)
Compare that with a chilled-out drift or gentle bimble where your SAC would be normal (15 l/min) so using 45 l/min at 20 metres, or 40 minutes of gas.
With more practice and relaxation, you can get your SAC down to 12 litres/min (or 50 minutes for 150 bar of a 12 litre cylinder).
Tech divers tend to use two SAC rates; the "bottom" SAC where you're moving around, and the "deco" SAC where you're hanginging around motionless whilst breathing rich oxygen mixes. For planning, I use a bottom SAC of 15 l/min and 12 l/min for deco, but know I use less.
For 'fun' I brought a half-empty deco stage with regulators into the lounge one night and measured the amount of gas I consumed whilst watching the telly and doing some internet browsing -- must have looked odd with a reg in my mouth and a clothes peg on my nose! In that 20 minutes, my SAC was 7 litres/minute.
So, in answer to your question; you'll need to be moderately fit. But the main thing is to be relaxed, which is much easier if you practice your skills, especially the art of doing nothing - buoyancy & weighting. The more you practice, the better you'll be.