Here's a little tidbit that might upset a few aikidoka - or maybe not...
Tsuki as it is practiced as an attack in class (similar to a Shotokan or TKD lunge punch) is a good place to begin learning aikido technique, but it tends not to happen that way in real attacks. What happens more often is more reminiscent of a jab-cross or grab-cross attack. While working against tsuki can teach you the mechanics that will allow you to deal with jab-cross and grab-cross attacks, you have to constantly keep the off hand in mind. It has to dictate pretty much everything you do in aikido.
If there's one lesson you learn in aikido randori, it is that if you become too intent on doing something to the lead hand, you will get smashed with the other hand. You have to be handling the rear hand as you are evading the lead hand and vise versa.
It can help you greatly if you will (at least every so often) explicitly practice your techniques or kata against the following three attacks:
- stab twice - rubber knife in the lead hand, uke gives one lunging attack from outside ma-ai like you're used to, but then uke can stab or slash again any way they want to. Uke's specific goal going into the technique is to cut you more than once no matter what happens.
- jab-cross - similar to the above. Uke leads with a jab then throws the rear cross. Either may be real or a feint for the next punch.
- grab-cross - practice grab defenses like the wrist releases with uke not simply stepping in and grabbing. Have uke grab with the goal of holding you in place or pulling you into a punch.
If you are unfamiliar with the jab or the jab-cross type attacks, here are some good sources for additional info: