If you are not totally convinced, take the next logical and least expensive step...
Buy a set of new or used toe clips and straps and install them on your pedals. See if you notice any changes in your performance.
Granted, tennis shoes are too flexible for efficient pedaling, but you'll have your feet more stabilized, be able to push forward and pull upward through the stroke. (Can't pull backward though)
For recreational riders, the cost and trouble of dedicated cycling footwear and clipless pedals aren't justifiable in their mindset..too few miles. Not everyone enjoys walking like a duck.
I started on toe clips, because clipless wasn't on the market yet. So, I acclimated to their feel, their advantages and disadvantages. Two major disadvantages that encouraged me to go clipless were lack of float and the need to reach down for the strap buckle. Once the cost of updating to clipless fell, (not all of us are obsessive or rich) I switched to a Look style system with float. The improvements are obvious. The egress is so much safer.
Bear in mind, the you can now find clipless pedals new for under $20 online. (ie. Nashbar sales)
SPD type systems as low as $16. That's about the price for new set of toe clips and straps!
Your real cost is in the shoes. But at least SPD clipless still allows you to walk normally, and pop on a pair of $5 platform inserts to use regular sneakers.
Given your aspirations for tough long rides, I'd go clipless now.