I walk 6 miles every day after taking my Citolopram, and have never heard or been told not to exercise after immediately after taking it. There would be a warning on the label if this were true like there is for not consuming alcohol while on it. Exercise is great for anxiety, I wouldn't put any emphasis on what you read.
I found absolutely nothing in the medical literature that contraindicates exercising after taking citalopram.
For further reassurance, speak with your doctor or local pharmacist.
For those of us with anxiety, the very worst place for us to get information is on the Net. To say there is a lot of wrong information out there is putting it mildly. In future, if you have questions, contact your pharmacist. They are, in my opinion, the best resource for information. They do not receive kick backs from the big pharmas like doctors do, so they have no vested interest in promoting one drug over another. And drugs are their business, whereas doctors really only know what the drug reps tell them. The reps also have a tendency to skip over the negative aspects of the pills they are pushing.
I trust the books I get my information from, but it's best to speak with your doctor/pharma before jumping back on your treadmill.
I believe that if exercising soon after taking this med would render it virtually useless, you would have been told by your doctor and the pharmacies would have to add a warning sticker to wait a specific amount of time before engaging in exercise.
Peace
Greenlydia