Sudden increases in volume seem to trigger both overtraining and increase the likely hood of injury. More is not always better even if the intensity is low. Allow yourself plenty of time to prepare for your event(s). If your event requires 5 hours of work to complete you may want to work backwards (several months) from your event date to determine a good start date.Maximum increase in training volume from week to week should not be more than 10% of the previous week's volume. If you're just starting out, start with a low volume of work and build from there. Establish consistency in training and gradually build up your volume. For example if you're looking to do a marathon and you longest run for the week is 30 minutes at best. This would be your starting point. Establish consistent training and then start adding your volume. 10% increase for 30 minutes would take you to 33 minutes. So, if you do the math you're looking at approximately 6 months and this is without recovery weeks or a taper. For a runner anticipating a 11:26/mile pace this is what they could expect to do to complete a marathon comfortably.
The disclaimer: Before starting a new exercise regimen you'll want to consult your physician. Be nice to yourself. Before you take a road trip you would have your car checked out right?