As GTC has stated, setting achievable but challenging goals is key. Sometimes it's more motivating to aim to run to a specific location, and not being concerned with the distance or the time. You may also want to take a look at the NHS' C25K plan.
My biggest motivation for running comes from Dean Karnazes' book, The Ultramarathon Man: Confessions of an all-night Runner. Give it a read, highly inspiring and will help tackle any motivational issues.
Aiming to run the Endure 24 again in June and have challenged myself to run 2013km before the end of the year, currently on 120km.
Not really a long distance runner (typically run 10-15km), but hoping to do at least a HM at some point later in the year also. Will be keeping in touch to read about training plans and advice scattered on here.