There are many different ways to go about this and these are simply my own thoughts on this subject and give a little insight into how I go about my planning process.
My basic training is split into easy, medium and hard weeks which I shuffle around to give a blend of quality training and appropriate recovery time to allow for long consistent blocks of training. If you are looking to step up your race distance, whether that is to a 25km, marathon or ultra distance, it is important to avoid injury through over training. By allocating certain weeks as recovery, you give your body time to adapt to the increased intensity or duration of training.
Each individual athlete needs to work out what their easy, medium and hard weeks would consist of with regard to number, type, duration and intensity of the sessions based on their experience and current fitness levels. As you are trying to increase the amount of time you spend on your feet, you should be looking to address this in your hard weeks with a long run, on trails, and follow this with an easy week to give time for recovery/repair. As your fitness increases, you will find that your long runs in hard weeks will become part of your medium weeks instead. For a starting point, a basic rotation of easy, medium, hard for six weeks will get you up and running. If you are more experienced, you can play around with the rota but don’t forget to put in those recovery weeks.
There are two other points to consider as part of your preparation for a longer (for you) trail race. Firstly, you need to realise the difference between road running and trail running. The trails require more balance, careful foot placement, constant core adjustments, different stride length, the ability to cope with sharp turns, steep climbs and a certain mental toughness; all of which can only be experienced out in the terrain. Train smart! Secondly, you will need to condition your quadriceps to cope with the extra hammering they will take from steep and long descents. Once again, this is best done by getting out on the trails and applying that training principle of specificity!