What kind of investing interests you, or do you feel suited for?
My job takes me 50-60 hours per week and is mentally exhausting - business analysis and computer stuff. So when I get home I don't feel like doing anything, even though I could.
I found it helpful to partner up so some tasks could be shared. Instead of spending hours mailing and calling for motivated sellers, we bought a contract from someone who had done that piece already (we were wholesaled to). We still got the house cheap enough so we could pay contractors to do the bulk of the work, while we put in the time we could afford on weekends to keep the contractor bills down a bit. Google docs allow us to share notes and keep updated and organized on costs, quotes, tasks...
I just try to take breaks of 5-15 minutes at a time and make phone calls and accomplish what I can during the day before my energy is drained.
And listening to investing podcasts and such on my commute, plus reading bigger pockets, books, and other sources, keeps me motivated and aware that many other people are finding success, and many starting out with less time or resources than myself!