well, it sounds like you already found out by watching how physical it can be. Of course, each dojo differs in how much of a workout you can receive, and you yourself can vary how much energy you put into it.
when i started Aikido i had been running every day, and for a while i did both each day, then as my desire (and need!) to practice at home increased, running eventually tapered off and then stopped. I'm in the USAF, and we have a fitness test each year, part of which is a heart rate monitored stationary bike test---kind of like a simplified stress test. i worried that i would not do as well as i used to do when i ran every day, the first time i took my bike test after going to all-Aikido workouts. Not only did i score as well as i always did (can't say better because it is in percentiles, and i was already at the top), but my legs hurt a lot less---guess the muscles needed to bike, while not the same as running, are similar to the ones needed for shikko.
i think weapons work has kept my arms in shape, although i still do pushups and weights, so hard to say.
finally, it has always been difficult for me to exercise to a point where i break a sweat---even on long road marches...but i'm drenched by the end of an Aikido class.
I hope you give it a try!