I agree with letmeoutofhere, it is difficult to recommend walks for the over 70's without knowing their capabilities. I am approaching 70 and have been walking on Dartmoor since I was a young child, so I am able to do quite strenuous walks across Dartmoor of around 20 miles, but many much younger people would not be fit enough to do this.
In addition to Richard Knight's excellent website which was recommended by letmeoutofhere, I would also recommend purchasing a few of the many walking guide books covering Dartmoor and also South Devon, as I see you have requested walks in this area in a separate post. I would suggest "50 Walks in Devon", which is an AA publication, and also the "Jarrold Short Walks" books publish one on Dartmoor and one on South Devon. All three of these books grade their walks into varying levels of capability.
Have you walked Drake's Trail? This is an ideal walk for those who may be less mobile as for most of it's length it follows the route of the former Plymouth to Tavistock railway line and is therefore fairly level. You can start the walk either at Yelverton or Bedford Bridge (aka Magpie Bridge) and finish at Tavistock. You can then make use of your bus pass and catch the No. 12 or 83 bus back to the starting point. Starting form Yelverton it's about 6½ miles or from Bedford Bridge it's about 4½. If starting from Bedford Bridge there is a short steep climb from the car park up to the old railway line. Where the trail crosses the Walkham valley you cross the newly built Gem Viaduct. It's about 80 feet high so you need a good head for heights when leaning over the railings to see the river below! Shortly after the viaduct you walk through the quarter-mile long Grenofen Tunnel. It is quite poorly lit but the ground has been well surfaced so there is no fear of tripping. You will get soaked from drips from the roof! On the approach to Tavistock you need to carefully follow the blue cycle trail signs which will take you to the town centre via the outskirts instead of the main road. One thing I should mention is that the trail is part of the National Cycle Network, so you need to watch out for cyclists. For this reason it is best to do the walk on a weekday when it is fairly quiet rather than a weekend when it can be quite busy.
I hope this helps. Enjoy your walking!