Hey Nina - good for you. Rowing will definitely keep you fit. Before I had kids I use to row and loved it. I started in an eight but ended up in a four as it was easier to coordinate training etc with less crew. We competed a little (masters events included) and also rowed the Murray River Rowathon which is a 92 km event down the Murray River.
Its a great sport.
Happy rowing and riding.
The most effective way to do it, is to do it.
Amelia Earhart
2005 Trek 5000 road/Avocet 02 40W
2006 Colnago C50 road/SSM Atola
2005 SC Juliana SL mtb/WTB Laser V