I'd go with more reps and increase the weight in
small increments as it becomes easier to do. But don't go too heavy. You don't want to
build up the muscles and make them bigger, which would defeat the purpose of slimming the thighs down.
I made that mistake trying to reduce my rear end. I was using a machine I thought would reduce my glutes and someone said to me, "you realize that the more you build up the muscles in your rear end, the bigger it will get." I quit using that machine immediately!
I considered not using the leg press too because that can increase the glutes, but I like using it to strengthen my legs. It's worth the trade-off.
I would look at working out as building a better, stronger body, not a weight loss solution. I used to work out when I was much younger and at that time it
did work. I think at 75 it no longer gives the same results. Personally, I work out because I want to still be walking around when I'm 90.