Sounds to me like your alternate picking is already very nice. If you're having some difficulty improving your speed, you may want to consider learning a more advanced style of picking, like directional picking. Despite the common misconception, alternate picking is not the best picking technique out there. And it's actually quite inefficient at times.
If you want me to explain directional picking to you, let me know. I'd be happy to break it down for you.
I usually practice scales and modes only about 10~15 minutes every other day or so. I actually spend a lot more of my time practicing things like improvisation. In fact, I only spend about a third of all my 'practice time' with a guitar in my hand. More of my time is spent with things like songwriting, ear training, theory, and analyzing music.
But here's a few quick tips for working on your speed:
This is pretty counter-intuitive, but the first thing to do is start slow. Painfully slow. Like 'metronome at 30 bpm and hit a note on each click' slow. While you do this, pay close attention to all the movements in your hands and be sure that they are as relaxed as possible and not making unnecessary motions. Tension and inefficiency are the things that will really slow you down
