Hi Suzanne,
As far as walks around Tampolo (Tampolodge, Forest Lodge) and the neighbouring village of Ambodiforaha (Arollodge) are concerned, there are "circuits" with a mixed level of difficulty, and you would be able to get into and enjoy the forest without too much trouble. The places where the red-ruffed lemurs are most frequently seen involve a moderate climb which gets a bit steep at one or two points, but I still find the going relatively easy on that particular trail.
At any rate, the guides are used to looking after people with different levels of fitness - for example some of the people who come to Masoala on upmarket packages are really quite elderly and frail ! - and you can discuss with them what you can and can't manage. There will certainly be rewarding walks for you. If you want to do a night walk at Masoala, your guide will normally select a pretty undemanding trail.
The kayak trips don't go into the forest but along the coast through the marine park.
On Nosy Mangabe, the walk to the Baie des Hollandais is more or less on the flat and is easy. If you want to visit the Malagasy tombs or the summit of the island, though, the trail is pretty steep and in places quite difficult, especially the descents, and especially if it's been raining, which of course it often has. When they're wet, many of the rocks and tree roots that form part of the summit trail become unpleasantly slippery, and it's not unusual for a tourist to take an occasional tumble on the way back down. You can see wildlife on both walks, though I must say that I've usually seen more on the trail up to the summit.
The standard night walk on Nosy Mangabe is flat, with the main difficulty being tree roots over which you can stumble in the dark (use your torch), and, depending how far you go, a couple of streams that you have to cross on stones. Again, the guides are always quite concerned to look after their clients and warn them of obstacles. The night walk is usually rewarding - you're almost guaranteed to see leaf-tailed geckos and nocturnal mouse lemurs, and there's always the question of will we/won't we see an aye-aye? (Most people, it must be said, don't.)
Hope that helps.
