This is a difficult question to answer without having walked with you!
Most people should be able to complete easy access, easy or leisurely walks without problems (unless they are very unfit). People with a good fitness level should also be able to do the moderate and easier strenuous walks without problems. However, it is recommended that you do not try the harder strenuous walks until you have completed several easier strenuous walks comfortably.
For your first walk, it is better to be cautious. If you try a walk that is too difficult for you, you will probably not enjoy it; further, it may be necessary for the walk leader to terminate the walk early, in which case the other people on the walk will not be happy either. It is much better to risk finding the walk too easy and trying a harder walk next time, than doing something outside of your comfort zone.
Note: walk difficulty is a combination of (in approximate order of importance) ascent, speed, terrain and distance. This is stated because often people look only at the distance when determining how difficult a walk is and this can be extremely misleading. If you compare a 12-mile walk with 1,500 feet of ascent done at a moderate 2 miles/hour to an 8-mile walk with 4,000 feet of ascent done at a fast 3 miles/hour, the latter will be much harder, despite being shorter.
Once again, if you are unsure, contact the walk leader and ask them what they think.