Normal advice is that an average active person should be aiming to do at least 3 half-hour long sessions which get you out of breath - per week. This is a standard recomendation to help your heart & general health. Naturally someone wishing to actively compete once they return to the water should be trying to do a bit more; although you can easily train to excess (you should have one proper rest day per week - not just away from the gym but away from excess heart rate increasing stresses too), but bear in mind that up to a point, the more you do the fitter you will become, and the faster the boat will travel next year!
Any form of active sport is good at this stage, providing you do it with a view for your fitness. (So, if you play football, don't end up being the goalie..........). However, not all of you may be able to participate in team sports regularly, so below are some suggestions of activities you can do to improve fitness, even in the middle of nowhere.
35-50 minutes continuous
(maybe put in 6-10 sprints during the run to relieve boredom.)
60-90 minutes continuous
(a mountain biking afternoon would count)
30-45 minutes continuous
(alternate stroke styles every 15 lengths)
5 x 5 minutes continuous
(it may take a few sessions to be able to skip that long)
50 minutes continuous
3 x 15. splitting < 2:05 (but aiming to improve this over several sessions)
5 x 5 minutes flat out (equal rest)
If you CAN get on the water:
60-90 minutes steady state
40 Deep Squats
3x40 Squat Jumps
3x40 Stand up/lie down/stand up
the above is continuous.
In between sets do the following:
15 press ups
15 burpees
15 angels
5 minutes light jog
at the end, do 10 minutes work on your abdominals (not optional)