Start small (no Bataan death marches) and let them stop and check things out - bugs, rocks, etc.  Kids love to touch and pick things up. For a young toddler, even a 100 yard walk on a park path can be a big adventure. Take time to point things out for them to investigate, tell them what they are and ask them about the things they see. The most important thing is to let them see you enjoying it too.