I would advise against TABBING in your own time mate. Its the most common cause of injury to Recruits in 4 Para.
Just get out and run, run, run!! The RRTT have Recruits TABBING every Sunday at the end of a training weekend and its progressive. Also when you deploy into the field your constantly on the move in full kit, so your body soon becomes accustomed to it.
Contact your local TAC and the PSI should be able to send you a training program. Alternatively get in on a drill night and speak to the PTI.