Hi Cymro,

It is doable but you will be looking at 1hr each way from the Hilton Tokyo Bay to Tokyo station,

5-10 mins from hotel to Bayside station
up to 5mins waiting for the train
10 mins to Resort gateway station
5mins to get to JR Mahiama statiom
and around 20mins to Tokyo station by train.

You might save a few minutes by getting the hotel shuttle bus direct to the JR station.
The JR line can be very busy at peak times.

I did it twice in Jan 2016 but in January 2017 I moved after 2 days to the Prince Sakura tower ( Marriott) in Shinagawa which was a lot more convenient. I would have liked to have stayed at the conrad but it was over £350 per night.
If you have Marriott or SPC status there are a few Marriott/SPG hotels in tokyo.

Regards

TBS