It's hard to say without seeing the LRF, but you could try doing the LRF to EPUB conversion with one of the following in the Calibre Extra CSS conversion box on the Look&Feel page.
Try each to see if either make a difference.
Code:
body {font-weight: normal}
or
Code:
p, div {font-weight: normal}
If neither work, you can remove the unwanted bold by using the Calibre Tweak-epub option to manually edit the epub's css file.