The fitter you are, the more you will enjoy it! You must be able to comfortably run 3km in under 25 minutes, plus complete a full day's physical activity.
There are varying fitness requirements for these courses:
Discovery/Navigator/Compass: Fast walk or run 3km in under 25 minutes
Aspire: Walk a minimum of 2kms and five flights of stairs (walking sticks ok).
For information on how to get fit for Outward Bound you can open this document Get Fit & Smokefree