What London underground tunnel is the longest between two covered stations?

The tunnel between East Finchley and Morden (via Bank) is the Underground's longest and one of the longest rail tunnels in the world, 27.8km (17.25 miles). However, Morden is covered by a canopy and is therefore tru to your question but East Finchley is not. The previous station to East Finchley is Highgate, approximately 1200 metres from East Finchley.