Do your research
Before building an appropriate training plan it is important to do your research in a couple of ways.
You should take the time to write down what you want to achieve and how you plan to do it, thinking through what time you can commit to, what forms of exercise you want to do as part of your training and what other factors will be important to manage (e.g. work commitments, family time etc).
This is similar to what you would do if you started working with a coach, in running or on pretty much any other discipline. The simple act of writing this down will make the goal and your steps to achieve it much more meaningful and tangible.
When building running programmes for athletes I use a simple Google form like the one below.
Research existing plans
There are countless examples of training plans in books and on the internet. I would recommend reading around until you find one (or more) that you like and start from there. You can then tailor it accordingly to meet your goals and limiting factors set out above.
Examples of plans (in order of influence on my own plan)
Early on in my training I used several of TheRunExperience programmes to build up running strength and a sense of consistency into my training. I like the focus on drills and running form and the incorporation of strength training. They advocate training 7 days a week (with suggestions about when to take a rest day if warranted) and it comes across as a measured approach. They only do plans up to marathon distance but this could easily be modified to scale up to 100 miles.
Book: The Art of Running Faster by Julian Goater
This is a great book to read to understand more about how to improve your running and what elements to incorporate into your plan. It boldly suggests incorporating days where you run twice (morning and evening) and to build lots of variety into your runs. It also introduced me formally to the idea of periodisation, namely setting phases throughout the year to be focused on different aspects of your fitness so that you reach peak form at the right time.
Website: Trailrunning.com 100 mile plan
This website has a relatively simple approach to training – don’t overdo it! I like the fact that the author focuses on consistency of a reasonable distance with 20 miles as the longest weekly run typically and a 50 mile or 100 km as a race preparation test 6-12 weeks out.
Book: Ultra-marathon Training by Wolfgang Olbrich
This book sets out 8-week training plans for races from 50km to 24-hours and beyond, so it assumes you already have a good baseline to work from. It was a bit of a wake-up call when I read it, but remember it’s just a guide. For instance, in training to complete a 100km race in under 10 hours it suggests your weekly distance to be from 80-139km. Looking through my Strava training log from 2017 I think my weekly distance peaked at about 50km! My training was complimented with at least two circuit sessions per week – something which is referenced in this book but not explicitly built in to the training plans themselves.