=They had a good/nice/great walk.他们的散步很愉快.
fine 在这里做adj,being satisfactory or in satisfactory condition 令人满意的,好的全句是这样的They had a fine walk too, which had done his liver good. 难道要翻译成: 他们也愉快地散了步，这对他的肝脏有好处？就是这么个意思， 有好处也可以翻译成有益， 主要这个句子太奇葩了，怎么翻译都感觉很怪。