A weight vest (or backpack with stuff in it) would be a good idea, since you used to wear a 65-lb "weight vest". Or you could choose walking routes with hills. But I would not use ankle weights for walking - they will alter your gait and could set you up for injury. Ankle weights can be used for more controlled exercises at home.

If you aren't worried about looking goofy, you could exercise your arms by walking with nordic poles - it gives a full body workout and using your arms makes your legs last longer.