Yes, you have gone too far, too close to the top of the legs. Lesson learned for next time, eh?
If I were in such a predicament, I think I would put some wood back into the mortice. A nice snug piece, with the grain in line. Properly done, it will make the leg as strong as it would have been if you had cut it like that in the first place.
Haunches are a good idea for that sort of thing, too, but a bit more difficult to do retrospectively. You would have to rout a short slot in the ends of the rail, insert a long-grain piece and take it from there.
You have made mistakes, but they are very remediable, and if this is your first attempt you deserve a pat on the back.
Steve, after the error was pointed out to me I did actually cut some new legs to final dimension but I decided to stick with the original legs (and hope nobody noticed!). I need a lot more joinery practice to produce cleaner more accurate M&Ts and the first attempt was just about OK so, I had a look at it and decided to glue in a block of wood in each which, apparently wasn't such a bad idea, thanks.