Acceleration is the key.
You're small which typically means quicker acceleration and changes in speed. (Simple physics: bigger objects require more energy to positively or negatively accelerate.)
I might not be the best person to advise you here but my suggestions are:
Do interval training which in layman's terms means run x distance while altering your speed now and then. E.g. run one kilometer and every 200 m switch between jogging and sprinting. Draw a small track with cones, sprint to the cone, brake, cut to the next cone, sprint and repeat. What you want is a quick pair of legs.
In the end, ultimate doesn't have that many long runs except hucks to the goal. Most attacks require a few steps to fake, an aggressive cut and an even speedier sprint for the open/break side to catch the disc.
But when defending, speed isn't all. You'll need to learn how to position yourself. That's more of a team effort, though. But you need to be able to read the play going around the field and put yourself in the attacker's position. Two guys run up for a huck, most likely your attacker won't so it might be more wise to position yourself between the thrower and attacker (under him). That gives you a slight edge as he'll need to work a hell of a lot more because
a. He's going to have to come through you if you position yourself well
b. You have a lot more reaction time to his movements
One thing about going backwards: keep your balance on your toes. Might wanna check out basketball players and their movement. Try not to cross your legs, that makes you slow and vulnerable. Attackers will try to push you to a position where your balance is on your the back of your feet. That's the point when they cut because you need time to get your balance back to your front before you can start accelerating.
And about the Japanese? They lay out every single time they feel like they can. Work on your jumps etc. you'll get a huge edge on being able to get your body low and jumping from behind and making that sweet grab.
And just to inspire you, here's a picture
from Finland. Just look at how much shorter the other guy is but he's still pretty even on air.