Throughout pre-selection you will be given practical and theoretical Navigation training with a map and compass in readiness for the hills.
You will be provided with a compass but buy a spare which uses mils not degrees. (Silva Expedition4 costs under £30)
As for map reading - basic map reading is fine- you have to start somewhere. But there is a great deal more to Navigation than just map reading. For instance, the ability to view a map and then cognitively translate the information into a realistic mental picture of the ground ahead is an important skill.
You've only got so much time for each leg, so you've only got so much time to keep checking your map.
The trick is to take a good look the map and soak it all up, then get on with tabbing (in the right direction!)
It's no secret the SAS spend most of their time in the hills. Navigation is a must, so JOIN AN ORIENTEERING CLUB NOW, and start looking at the Landranger series of maps, understand how the cartographer depicts things on it, learn spacial awareness, learn to interpret the ground to the map and the map to the ground.
Three dimensial interpretation of what the cartographer has drawn in relation to what you see is imperative.
Familiarise yourself with best routes of travel, stay close to paths, (its a safe bet that whilst it may not be the shortest route, it will be the quickest under foot) stay clear of short cuts, marshes, scree, cliffs and rivers and streams.
Stay to the high ground and wind your way around the paths to your next RV that you have set yourself.
Routes for selection will always start at the bottom of the lowest point MSL and end up at the highest point MSL.
You must get used to carrying a bergan over rough terrain up and down hills all day.
Navigation can be worrying but dont let it get to you. The more you do it the easier it gets, so get out on the hills and start it off easy- a little bit of pacing to a ring contour. Check your pacing over different ground.
Dont treat the day on the hill as a hard core tab, that way you give 100% to your nav and not the fitness, as that will come whilst your naving.
Try and devote equal time to 1:25k as a 1:50k map.
One handy tool for training is a GPS- never rely solely on one for 'live' nav, but for training, it's a quick and handy way to confirm your manual navigation skills (saves a lot of time, too). Having said that, it takes discipline to avoid using a GPS when you should be relying on your manual skills.
If you have a GPS dont be afraid to use it to check your position. Do a few short micro nav lags of no more than 300m per leg, read off your 6/8 fig co-ordinates and then get the GPS out- you may be suprised how close you are, and this is a real big confidence booster.
Be aware that maps are not always accurate when used with a GPS- you may find buildings and streams in the wrong place on the map when using a GPS! Its bad enough that OS mix up walls with streams and vice versa when they plotted the maps from aerial survey all those years ago!

Useful reference books include-

Land Navigation by Wally Keay
Mountain Navigation by Peter Cliff
Mountaincraft & Leadership by Eric Langmuir