Swimming is the best all round exercise so I would go for that. There is no point in toning up the bottom half if the top half isnt equally ship-shape.
Otherwise try treadmills or skiing machines for just the lower part of your body.
You need to do at least 45mins exercise in order to start burning off calories and toning up, as it takes about 15 to 20 minutes to warm the muscles up sufficiently in the first place.
However, as with all exercise there is NO overnight success. You are looking at approximately 4 to 6 weeks before a significant difference can be seen.