Of course you can, if you really want to.
Ten years ago marathoners were considered to be members of an elite club. Nowadays people look at ultramarathoners and ironmen that way. I'll let you in on a well kept secret within the ulramarathoners club: Running a slow ultra is easier than running a fast marathon.
The key to success is changing your state of mind and slowing the pace, instead of sisifusly attempting to shave off some minutes from your marathon time.
For those of you who are thinking- how can he be so sure about our abilities without knowing us or our shape? I will say that as long as you do not have a severe disease, your human spirit can push your sorry ass to the finish line. I've competed with athletes who have no legs, so no excuses please.
Another common excuse is that you don't have the time. First of all, it's a matter of priorities- so if it's important to you, you'll find the time. Second of all, many people assume that you need to train at least 15 hours per week, but 10 hours a week is enough for a 50k ultra.
If you're calculated and efficient, you can plan your week so that your training will not throw your life out of balance. For example, run circles around your kid's soccer practise, maintaining eye contact, while getting in a training session. Run to work or run along your kids while they go biking. One of the accomplishments I'm most proud of is finishing my first Ironman competition while my three kids were 7, 4 and six months old, while still being there for my family during that period.
So, decide on a goal/race that you want to aim for, put it on your calandar, and off you go!