As a starting point you can walk to Restormel castle and then into Lanhydrock, a very popular National trust house and garden. You can get the train back from Bodmin parkway if you don't fancy walking both ways (although I'd check the timetables first - not all trains stop in Lostwithiel, so you may want to start with the train and walk back.
For some ideas & routes, see https://www.lostwithiel.org.uk/walks-and-paths/
The walk to Lanhydrock (& back the way I came) is my longtime favorite walk starting in Lostwithiel. I can't count the number of times I've done it.
I've also done the Milltown walk four or five times (pleasant but nothing spectacular); Lostwithiel to St Winnow (but not onward to Lerryn) once; and the Duchy of Cornwall Nursery loop (or some variation of it) a couple of times.
The Hall Walk (starting from Boddinick, across the River Fowey from Fowey) is another all-time favorite.
https://www.nationaltrust.org.uk/fowey-estuary-to-pencarrow-head/trails/fowey-hall-walk
In fact, there are a number of excellent walks beginning in Fowey or across the river in Polruan. Lots of information here:
http://www.iwalkcornwall.co.uk/walks/near/fowey
http://www.fowey.co.uk/activities-and-attractions/walking
http://www.westcountrywalks.com/mid-se-cornwall/cornwall-secorner/fowey-polruan/fowey-polruan.php
http://www.cornwall-online.co.uk/walking/walks/fowey-gribbon.htm
Contributor 
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7 helpful votes The nearby Luxulyan Valley is part of the Cornish Mining World Heritage SIte, some great walks, spectacular structures, and interesting archaeology. Walks are documented on Jack's Iwalkcornwall link, and via the Friends web-site.
http://www.luxulyanvalley.co.uk/interactive-map/
https://www.cornish-mining.org.uk/areas-places-activities/luxulyan-valley-charlestown
Thanks to all
