There's really no such thing as too much, if you enjoy your walking and you're matching or exceeding the government recommendations then continue if you want to. These recommendations give "important health benefits" so you're doing yourself good. However, you can go further if you want to and I don't mean walking further. Once you go past 30 minutes then you may want to try something new.
Walking 30 minutes 5 days a week should have lost you a lot of weight and put you in reasonable condition. What I mean when I say go further is try and add some jogging or running into your routine. There are ways to put it in gradually. By jogging the CDC class this as vigorous physical activity and you only need to do this for 1 hour and 15 minutes per week or 15 minutes per day, 5 days a week.
Back to gradually introducing jogging into your daily workouts. There is a walk/run way of doing it. This involves walking for a minute and then jogging for a minute and repeating that procedure 5 times. This is something you should be able to do even if you haven't run for years. That's what the plan is designed for. I have a web page with one of the plans and it's made for the treadmill and it's the couch to 5k.
That couch to 5k will have you building up your jogging time each week and in the end, after 9 weeks, you'll be jogging for 30 minutes non stop! The speed you jog is not important at all, as long as you're lifting your feet off the ground at every stride you're good. People so often run too fast and not achieve the final weeks, slow and steady wins the race on this one.
Remember if you're happy treadmill walking then there is no such thing as too much and you can keep doing it. If you're getting what you want out of it then ignore people who criticize or tell you to do something else. I do all my running on a treadmill and none outside and I get called on it all the time, but I enjoy it and that's what counts.
If you're finding it a bit boring and you've tried the incline and want something more. If you want to lose more weight and have reached a plateaux with the walking. Then jogging is something you may want to consider, it's just as easy as walking you just have to take the step and there's plenty of routines out there to help you make the transition. Just keep enjoying your treadmill exercising.