Still not sure I'm pushing enough

Today was my last day of strength training for the week. I walked 1.35 miles at full incline in 30 minutes. Seeing myself go almost as far walking at a full incline as I did running definitely makes me feel like I haven't been pushing enough. I will have a better gauge after my running workout next week.
I have found that it's really easy for me to skip a workout. I didn't get a chance yesterday and I didn't even miss it. The whole "it takes 21 days to form a habit" thing has never worked for me. I can workout for months and stop and not miss it one bit. I have to tell myself every day that I need to workout. It will be a lifelong battle for me I think.