There are loads of lovely walks between Plymouth and Tavistock and out of Tavistock towards Cornwall or Okehampton so all you need to do is to check the bus time table and off you go. 
The Stagecoach No1 travels from Plymouth Royal Parade to Tavistock generally every 20 minutes except on Sundays.
The 118 from Tavistock to Okehampton and back is an hourly service.
The 79/87 service which will take you to the Bere Peninsular  and  Tamar Valley walks is also approximately hourly. 
Click on the walk name to download the walk. 
 
Walk 1 Crownhill to Roborough via Tamerton Foliot  

Starts from: Crownhill low level bus stop.

Distance: 3.5 miles or 6.5 miles if you continue to Roborough.

Difficulty: Very easy walk much of it through woodland.

Description: I have called it a countryside city walk as the majority of the walk is through woodland. When you reach Tamerton you can catch the 42 back to the city or go on to Roborough which is again a pleasant walk mostly through woodland. You can catch the No.1 or City bus 42 from Roborough.

Walk 2 Woolwell to Derriford Hospital

 Starts from: near Tesco stores Roborough

Distance:  5.7 miles

Difficulty: A moderate walk with a few ups and downs

Description:  A linear walk from near the  Tesco Store to Derriford Hospital and takes you via the river Plym. A large part of the walk is on an ancient tramway that is also used in Walk 3. There is hardly any road walking despite going through the Estover estate.  (Temporarily rerouted to avoid the roadworks now starts in Longpark Drive off Woolwell road)

Walk 3 Around Roborough Down 

Starts from: The next bus stop after passing Strawberry Fields shop on the edge of the moor. Or if coming from Tavistock the next stop after passing the left turn to Clearbrook

Distance: about 7 miles

Difficulty: An easy walk with a long near level section on the old railway line now a cycle track.

Description: A day out on the edge of the moor. You  get to walk through an old railway tunnel, along an even older tramway and get to see what some believe to be an iron age  hill fort just 50m from the A386 that you probably never knew existed.

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Walk 4 Yelverton to Clearbrook

Starts from: Bus stop opposite Yelverton shops

Distance:  4.5 miles.

Difficulty: easy  circular walk.

Description: A popular circuit from Yelverton that  uses the ancient tramway and  the route 27 Cycle track. It will take you  right past the Skylark pub and down to the river Meavy. The return to Yelverton is via the Harrowbeer wartime airfield and Roborough Rock.

Walk 5

Horrabridge to Grenofen  The Three bridges walk

Starts from: the Horrabridge Bus  stop on the A386.

Distance: 3.5 miles

Difficulty: A moderate walk with a few ups and downs

Description: A linear walk that  takes in Drakes Trail cycle track and a path by the river as well as going under or over 3 bridges including the magnificent Gem bridge, built in 2012, that spans the Walkham river.

Walk 6 Anderton Lay-by Whitchurch Tavistock to Double Waters

Starts from: The bus stops by  the large Anderton lay-by on the south side of Tavistock on both sides of the  A386.

Distance: 5 miles but can be extended to 6 miles.

Difficulty: Moderate walk into and out of the Walkham Valley

Description: Its a circular walk that takes you on Drakes Trail through Grenofen Tunnel over Gem bridge and the newly replaced (2023) bridge at Double Waters and can be extended to walk besides the river Walkham.

Walk 7 The Harvest Home at Gulworthy to Tavistock

Starts from: The Harvest Home ( see photo on the left) once a pub on the A390 Tavistock to Gunnislake road. 

Distance: About 3 miles

Difficulty. Very easy walk as its either downhill or level. 

Description: This linear walk takes you all the way along the Tavistock canal. To reach the start Catch the 79 or 87 at Tavistock bus station and get off at the Harvest Home bus stop and walk back to Tavistock Bus Station an almost entirely off road walk just crossing 3 roads but not walking along any. The bus only rarely stops here so tell the bus driver where you are going when you get on.

Walk 8 Gunnislake Mines and manure 

Starts from:  Bus 79 Tavistock to Callington. Get off by the Gunnislake railway station

Distance: 4.25 miles

Difficulty: A moderate walk with a few ups and downs

Description: A circular Tamar Valley walk from the Railway station  You can either catch the 79 from Tavistock or the train from Plymouth. The walk takes you through a lovely wood past many mining features as well as a long since disused canal and lock.

Tavistock to Okehampton
 
The 118 that goes between Tavistock and Okehampton is an hourly service calling in at  Mary Tavy North Brentor Lydford and Sourton. 
Check out the walks along the WEST DEVON WAY  and in DARTMOOR Middle and North. For instance you could go on WDW Walk 10 from Mary Tavy and Walk 11 from Lydford both of which go through the middle of the village. Have a look at The Lydford Viaduct walk it doesn’t start in Lydford but it is only a short distance to reach this circular walk.
Starting at Okehampton Station you can go on WEST DEVON WAY  Walk 15 that goes past the station and also the Irishman’s wall under Dartmoor North Walks. The walk starts at the station.