You can take "trail walks" instead of riding. I love doing that with my youngsters. Good exposure for them, exercise for me.
The biggest thing is changing it up so they don't lose interest and get bored. You could make a trail course/obstacle course and conquer the horse eating monsters together. Mailbox, rain slicker on a barrel, "L" to back through, bridge (sheet of plywood makes a quick one), working a gate, tarp to walk over, etc.
Even after I've got mine under saddle, I like to ground drive them or drag logs on off days.
Ground work, you can never do too much ground work. Yielding fore & hind, sidepass, pivots, etc.
Life is like a camera. Focus on what's important, Capture the good times, Develop from the negatives and if things don't work out, Take another shot.