Ring your local unit right away to register your interest and they will give you a specific training programme and any advice you need.
In the meantime..

Month 1
Just run. No weight- and dont worry about the speed, get used to the distance. Get up to 4 miles. I would start doing low & slow press ups as well, gym if you want but press ups or core strength. These will also start to get some strength into shoulders for when that bergan tries to rip your shoulders apart!!

Month 2&3
Just run 4 times per week, no weight again. Resist the temptation as if you run with weight, you will shag your knees or worse shin splints. Make sure that you're buying new trainers every 10 weeks or new shock insoles as well. Believe me it helps massively. Dont forget the gym/swim & press ups

Month 4
Best trainning for being on the hills is getting on the hills! Day sack, little weight and 50% beasting- just get used to it. Keep up your running 4x per week as follows (2x 5/6 milers fast, 2 x 8 milers medium pace). Did I mention pressups?!!

Month 5
Keep on running! 4x per week
2x 5/6milers on grass with say 20lbs
1x 6/7 miler - fast
1x 8 miler - getting your times faster - 64mins
Plus hill work with say 30lbs ish. Press ups!

Month 6&7
Running & gym work
1x grass weights session 6/7 miler 30lbs
1x 8 miler fast
1x 10 medium
1x Upper body (shoulders especially) workout
Plus some hill work and of course lots of lovely press ups (I love press ups!)
3 days before chill out, light swim, pasta etc.

Phone your local unit and get yourself on the next selection, you will be in good fitness for it!
It's worth noting its getting the right balance between intensity and rest more than getting the weight on your back all the time.

Here's one week off the build up programme for uksf candidates
Monday , BPFA
Tuesday, 20 min swim trousers/shirt
Wedensday,run/march hilly terrain 45lb 2-3 hours
Thursday, circuit training 12 exercises 20 reps max 3 circuits
Friday,60min run 60-70%mhr
Saturday, 40 mins flexibility
Sunday,run/march hilly terrain 30lb 2-3 hours
thats a typical week over the three month build up phase the pre aptitude (start 3 months before selection)
A typical week goes
Monday Am,bpfa Pm 20 min swim
Tuesday Am, 30 min run 60-70% Pm 40 min run same
Wedensday Am rest ,Pm run/march hilly terrain 40lb 2-3 hours work on navigation
Thursday Am rest Pm run/march 45lb 10 x 1 hill reps
Friday Am circuits 12 exercises 20 reps 4 circuits Pm 50 min run 70-80%mhr
Saturday Am 40 mins flexibility Pm rest
Sunday run/march hilly terrain 45lb 2-3 hours work on navigation

So you can see where intensity jumps up but is still progresive.

Fitness Programme

This programme is not endorsed by any UKSF unit.

Note: This programme assumes you already have a good level of fitness!

Month One and Two
Day
Activity / Duration
Notes

Monday
Easy run 5 miles
Aim to increase distance every 2nd week for stamina and endurance purposes
50/60% of HMR (max heart rate)

Tuesday
Rest
Let your body recover and become stronger!

Wednesday
Swimming easy 20 mins
Aim to increase some faster lengths
Could mix with weights session afterwards

Thursday
Hard run 3 miles
Aim to increase distance every 2nd week
“Best effort”
80/90% of MHR

Friday
Rest

Saturday
am - Run a BFT
pm - Weights session
1st - 1½ miles - 14mins jog and walk
2nd - 1½ miles - Fast as you can !
Aim for 3 x sets of 15 reps
Try and increase weight gradually

Sunday
Tabbing over hills
20lbs rucksack - aim for 10 mile walking (take necessary gear for emergencies…)

Notes :
This is not a rigid plan, as similar training plans can become very boring - do what you feel like but always aim for a minimum. Battling through is often good for confidence.

If you have a bad day in the hills during selection remember the effort you have put in during training and take heart from it. Find a training partner or join a running/tri club - always push yourself as the DS on the selection are guaranteed to push you!

Food and drink-
EAT AND DRINK CRAP AND THAT IS HOW YOU WILL PERFORM!

Month three and four
Day
Activity / Duration
Notes

Monday
Easy run 8-10 miles
For stamina 50/60% of HMR (max heart rate)

Tuesday
Rest

Wednesday
Swimming easy 20 mins
Swimming hard 5 - 10 mins
Treading water for 5 minutes
Could mix with weights session after

Thursday
Hard run 5-8 miles
Best effort 80/90% of MHR
Aim for 8 miles in 60 mins by 1st we/end (or 7 1/2 min miles)

Friday
Rest

Saturday
am - Run a BFT
pm - Weights session
1st - 1½ miles - 14mins
2nd - 1½ miles - Fast as you can !
Aim for 3 x reps of 15 maximum
Try and increase weight gradually

Sunday
Tabbing over hills
40lbs rucksack - aim for 15 mile walking (take necessary gear for emergencies…)
Target is to average 4 km/hr for the entire distance travelled.