You may have two problems. Your sabumnim identified that you need to close your distance more; yet you felt that you lack strength. I can say that strength is not the reason you hit with toes instead of instep - that is definitely an issue of distance, so I do agree with sabumnim.
That said, you might have other manifest problems that have much to do with strength at your ranges of motion. To resolve this, you need to work on isometric exercises. These both increase your flexibility as well give you strength at your extreme ranges of motion.
As an example of one exercise, and there are many, see how this works, and then apply it to different kinds of kicks:
Stand with your right hand against a wall, elbow slightly bent. Normally, I'd have feet parallel; but in this case, since we're working on strength at greatest range of motion, I'd suggest opening the foot more to allow greater height.
Now, lift your left leg to the side (9:00 direction) as high as you can. With a partner, have the person retract your foot while you resist that retraction. They need only retract about 6 inches or so.
Similarly, do the reverse: at the greatest range of your motion, have the person resist your attempt to bring the leg back down to standing position.
You can repeat this with a lift in the 12:00 position, and at the oblique directions (10:30 and 7:30 positions). Similarly, do this for the other side as well.
Repeat these sets several times; be careful, these muscles are already compromised, and you can easily hurt yourself. Take it easy at first, and then increase the resistance as you feel comfortable. If you are young, give a day or so rest before trying again. If you are older, give yourself another day's rest.
As a way to change things up, you can use bands or ankle weights. I'm not a real fan of these only because the bands can snap (as has happened to me for the umpteenth time just yesterday), and when it snaps, that potential energy gets quickly converted into kinetic energy, and you can seriously injure yourself there.
And my problem with the ankle weights is this: if you can lift over 120 degrees standing, that is, just over your shoulder, then those weights are not offering any resistance as the downward force is being absorbed by your skeletal system.
However, if you can lift no more than 120 degrees, and more than 45 degrees, then you're getting best benefit using the ankle weights.
Finally, if you're really out of shape, and cannot lift more then 45 degrees, then ankle weights are absorbed by gravity and are not providing any resistance at all.
