The campaign will include a searchable database on the National Trust's , and a guide to the UK's best walks, with maps and directions.
The sponsorship stemmed from the companies' common interest in the over 50s market, and will include Rias's branding featuring in the National Trust magazine, member publications and all marketing materials.
Janet Connor, managing director of Rias, said: "Our research showed that walking is the main leisure pursuit for over 50s and so this new initiative is a great fit for Rias.
"The walks planned by the National Trust combine beautiful scenery with the benefits of getting out and about in the great outdoors, perfect for our customers who I know will relish such an opportunity."
Jo Burgon, head of access and recreation at the National Trust, said: "We have a vast range of spectacular walking opportunities and the sponsorship from Rias will enable us to promote these to a wide audience."
Genesis Partnerships brokered the deal and will manage the relationship between Rias and the National Trust, which has 3.5m members and 49,000 volunteers.